Is the address of a local variable a constexpr?
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In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):
extern char glob;
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
When I run this, I get:
error: ‘(const char*)(& loc)’ is not a constant expression
Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?
c++ c++11 constexpr memory-address
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
|
show 2 more comments
In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):
extern char glob;
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
When I run this, I get:
error: ‘(const char*)(& loc)’ is not a constant expression
Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?
c++ c++11 constexpr memory-address
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
2
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
2
char locis a locally declared character that is not static. The next timef()is called there is no guaranteelocwill have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf
– David C. Rankin
1 hour ago
1
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
2
Yes, I was kinda scratching my head as to where&globcame from, but&locwas identifiable.
– David C. Rankin
58 mins ago
|
show 2 more comments
In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):
extern char glob;
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
When I run this, I get:
error: ‘(const char*)(& loc)’ is not a constant expression
Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?
c++ c++11 constexpr memory-address
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):
extern char glob;
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
When I run this, I get:
error: ‘(const char*)(& loc)’ is not a constant expression
Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?
c++ c++11 constexpr memory-address
c++ c++11 constexpr memory-address
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
edited 56 mins ago
johnnyodonnell
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
asked 1 hour ago
johnnyodonnelljohnnyodonnell
378114
378114
2
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
2
char locis a locally declared character that is not static. The next timef()is called there is no guaranteelocwill have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf
– David C. Rankin
1 hour ago
1
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
2
Yes, I was kinda scratching my head as to where&globcame from, but&locwas identifiable.
– David C. Rankin
58 mins ago
|
show 2 more comments
2
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
2
char locis a locally declared character that is not static. The next timef()is called there is no guaranteelocwill have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf
– David C. Rankin
1 hour ago
1
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
2
Yes, I was kinda scratching my head as to where&globcame from, but&locwas identifiable.
– David C. Rankin
58 mins ago
2
2
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
2
2
char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf– David C. Rankin
1 hour ago
char loc is a locally declared character that is not static. The next time f() is called there is no guarantee loc will have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf– David C. Rankin
1 hour ago
1
1
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
2
2
Yes, I was kinda scratching my head as to where
&glob came from, but &loc was identifiable.– David C. Rankin
58 mins ago
Yes, I was kinda scratching my head as to where
&glob came from, but &loc was identifiable.– David C. Rankin
58 mins ago
|
show 2 more comments
3 Answers
3
active
oldest
votes
Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:
constexpr const char* p1="asdf";
This is OK because "asdf" is stored in a fixed memory location.
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.
answered 1 hour ago
dougdoug
8621410
So, you're saying that Bjarne should not have said that&locwill be "OK", right?
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
add a comment |
It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.
The example appears to have been updated in some pdf versions as seen here:
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
add a comment |
Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;
It's worth noting that:
string-literals have static-storage duration:- Based on constraints in declaring
externvariables, they'll inherently have static-storage duration or thread local-storage duration.
Hence this is valid:
#include <string>
extern char glob;
std::string boom = "Haha";
void f(char loc) {
constexpr const char* p1 = &glob;
constexpr std::string* p2 = nullptr;
constexpr std::string* p3 = &boom;
}
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
add a comment |
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3 Answers
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active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:
constexpr const char* p1="asdf";
This is OK because "asdf" is stored in a fixed memory location.
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.
answered 1 hour ago
dougdoug
8621410
So, you're saying that Bjarne should not have said that&locwill be "OK", right?
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
add a comment |
Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:
constexpr const char* p1="asdf";
This is OK because "asdf" is stored in a fixed memory location.
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.
answered 1 hour ago
dougdoug
8621410
So, you're saying that Bjarne should not have said that&locwill be "OK", right?
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
add a comment |
Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:
constexpr const char* p1="asdf";
This is OK because "asdf" is stored in a fixed memory location.
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.
answered 1 hour ago
dougdoug
8621410
Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:
constexpr const char* p1="asdf";
This is OK because "asdf" is stored in a fixed memory location.
void f(char loc) {
constexpr const char* p0 = &glob; // OK: &glob's is a constant
constexpr const char* p2 = &loc; // OK: &loc is constant in its scope
}
However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.
answered 1 hour ago
dougdoug
8621410
answered 1 hour ago
dougdoug
8621410
answered 1 hour ago
dougdoug
8621410
answered 1 hour ago
dougdoug
8621410
8621410
So, you're saying that Bjarne should not have said that&locwill be "OK", right?
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
add a comment |
So, you're saying that Bjarne should not have said that&locwill be "OK", right?
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
So, you're saying that Bjarne should not have said that
&loc will be "OK", right?– johnnyodonnell
1 hour ago
So, you're saying that Bjarne should not have said that
&loc will be "OK", right?– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
The example code I used in the question is taken verbatim. After looking at @doug's pdf, I think the hard-copy book that I own is incorrect. I think this mistake was updated in later versions.
– johnnyodonnell
1 hour ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
Is it the same as this? github.com/boydfd/books/blob/master/seeing/stalled/…
– jackw11111
59 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
@jackw11111 my version is different from the pdf provided in that link. doug provided a link to the same pdf. My hard-copy and that pdf show different examples for section 10.4.5
– johnnyodonnell
58 mins ago
add a comment |
It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.
The example appears to have been updated in some pdf versions as seen here:
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
add a comment |
It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.
The example appears to have been updated in some pdf versions as seen here:
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
add a comment |
It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.
The example appears to have been updated in some pdf versions as seen here:
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.
The example appears to have been updated in some pdf versions as seen here:
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
answered 52 mins ago
johnnyodonnelljohnnyodonnell
378114
378114
add a comment |
add a comment |
Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;
It's worth noting that:
string-literals have static-storage duration:- Based on constraints in declaring
externvariables, they'll inherently have static-storage duration or thread local-storage duration.
Hence this is valid:
#include <string>
extern char glob;
std::string boom = "Haha";
void f(char loc) {
constexpr const char* p1 = &glob;
constexpr std::string* p2 = nullptr;
constexpr std::string* p3 = &boom;
}
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
add a comment |
Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;
It's worth noting that:
string-literals have static-storage duration:- Based on constraints in declaring
externvariables, they'll inherently have static-storage duration or thread local-storage duration.
Hence this is valid:
#include <string>
extern char glob;
std::string boom = "Haha";
void f(char loc) {
constexpr const char* p1 = &glob;
constexpr std::string* p2 = nullptr;
constexpr std::string* p3 = &boom;
}
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
add a comment |
Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;
It's worth noting that:
string-literals have static-storage duration:- Based on constraints in declaring
externvariables, they'll inherently have static-storage duration or thread local-storage duration.
Hence this is valid:
#include <string>
extern char glob;
std::string boom = "Haha";
void f(char loc) {
constexpr const char* p1 = &glob;
constexpr std::string* p2 = nullptr;
constexpr std::string* p3 = &boom;
}
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;
It's worth noting that:
string-literals have static-storage duration:- Based on constraints in declaring
externvariables, they'll inherently have static-storage duration or thread local-storage duration.
Hence this is valid:
#include <string>
extern char glob;
std::string boom = "Haha";
void f(char loc) {
constexpr const char* p1 = &glob;
constexpr std::string* p2 = nullptr;
constexpr std::string* p3 = &boom;
}
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
answered 35 mins ago
WhiZTiMWhiZTiM
18.1k33153
18.1k33153
add a comment |
add a comment |
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Required, but never shown
2
Clearly, &loc can't be a constexpr. However, these lines of code don't appear on my kindle version. He does show a constexpr for the address of a "C" style string in a local function. That's legal since these are in global space while loc is an argument on the stack and not constant. Is that example what you are referring to?
– doug
1 hour ago
@doug check section 10.4.5 Address Constant Expressions. I'll update the question to make this more clear. Also, my example is abridged
– johnnyodonnell
1 hour ago
2
char locis a locally declared character that is not static. The next timef()is called there is no guaranteelocwill have the same address. 10.4.5 makes that distinction between an address assigned by the linker and those assigned by the compiler. 2013 Stroustrup - The C++ Programming Language 4th Edition.pdf– David C. Rankin
1 hour ago
1
@DavidC.Rankin it looks like your version (a pdf) is different from mine (a hard-copy). This must have been a mistake and was updated by the time your pdf was created.
– johnnyodonnell
1 hour ago
2
Yes, I was kinda scratching my head as to where
&globcame from, but&locwas identifiable.– David C. Rankin
58 mins ago