use of constexpr in header file

问题

I can have a definition like this in a header file?

 constexpr double PI=3.14;

Is there any problem in having this in a header file that would be included to several cpp files?

I am worried that since it says in standard that this constexpr has its own memory, putting it in header, and adding header to several cpp files, generate multiple copies of the same value in memory and some other nasty problems.

I am using C++11

回答1:

constexpr implies const and const on global/namespace scope implies static (internal linkage), which means that every translation unit including this header gets its own copy of PI. The memory for that static is only going to be allocated if an address or reference to it is taken, and the address is going to be different in each translation unit.

That implied static for const variables was introduced specifically to use const instead of #define in header files in C++ to define constants. Without static there would be multiple symbol definitions linker error if that header file is included in more than one translation unit which were linked together.

In C++17 you can also make it inline, so that there is only ever a single copy of PI if an address or reference to it is taken (i.e. not static). inline variables were introduced in C++17 to allow for header-only libraries with non-const variable definitions in the header files. constexpr on static data members implies inline, so inline is unnecessary there.

In other words, you should use constexpr for your constants in header files, if possible, otherwise const. And if you require the address of that constant to be the same everywhere mark it as inline.

回答2:

In C++17 you are clear. In C++11, you can wrap it in a function:

constexpr double PI () { return 3.14; }

回答3:

C++17 inline variable runnable example

C++17 inline variables were mentioned at: use of constexpr in header file and here is a minimal runnable example that shows that only a single memory location is used:

main.cpp

#include <cassert>

#include "notmain.hpp"

int main() {
    // Both files see the same memory address.
    assert(&notmain_i == notmain_func());
    assert(notmain_i == 42);
}

notmain.hpp

#ifndef NOTMAIN_HPP
#define NOTMAIN_HPP

inline constexpr int notmain_i = 42;

const int* notmain_func();

#endif

notmain.cpp

#include "notmain.hpp"

const int* notmain_func() {
    return &notmain_i;
}

Compile and run:

g++ -c -o notmain.o -std=c++17 -Wall -Wextra -pedantic notmain.cpp
g++ -c -o main.o -std=c++17 -Wall -Wextra -pedantic main.cpp
g++ -o main -std=c++17 -Wall -Wextra -pedantic main.o notmain.o
./main

GitHub upstream.

The C++ standard guarantees that the addresses will be the same. C++17 N4659 standard draft 10.1.6 "The inline specifier":

6 An inline function or variable with external linkage shall have the same address in all translation units.

cppreference https://en.cppreference.com/w/cpp/language/inline explains that if static is not given, then it has external linkage.

See also: How to declare constexpr extern?

Tested in GCC 7.4.0, Ubuntu 18.04.

C++20 std::math::pi

Note that for the specific case of Pi, C++20 offers a dedicated variable template as shown at: How to use the PI constant in C++

回答4:

I can have a definition like this in a header file?

Yes

Is there any problem in having this in a header file that would be included to several cpp files?

A constexpr variable (int, double, etc) in do not occupy memory, thus it does not have memory address and compiler handles it like #define, it replaces variables with value. This is not true for objects though, that is completely different. Read this:

To elaborate on comments made. To avoid overhead, in most cases constexpr is replaced with its value, but in cases where you have to get an address of constexpr compiler does allocate memory each time. So if you have ab.h which contains:

constexpr double PI = 3.14;

and you have a.cpp which contains:

std::cout << PI << "\n";

PI would be replaced no memory would be allocated.

Where as if you have b.cpp:

double *MY_PI = &PI;

memory would be allocated specifically for that instance (or maybe for entire b.cpp file).

EDIT: Thanks to @HolyBlackCat and his code he had in comments bellow it seems that memory is allocated per file.

EDIT 2: it is file based. So I have constExpr.h containing follwoing:

#ifndef CONSTEXPR_H
#define CONSTEXPR_H

#include <iostream>

constexpr int a = 5;
void bb ();
void cc ();

#endif

a.cpp containing follwing:

#include <iostream>
#include "constExpr.h"

void aa () {
    std::cout << &a << "\n";
}

int main () {
    aa ();
    bb ();
    cc ();
    return 0;                                                                                                                 
}

and b.cpp containing following:

#include "constExpr.h"

void bb () {
    std::cout << &a << "\n";
}

void cc () {                                                                                                  
    std::cout << &a << "\n";
}

output is:

0x400930
0x400928
0x400928

CONCLUSION But, honestly I would never do something like I did in my examples. This was a great challenge for me and my brain. constexpr is added mostly to replace #define. As we know #define is hard to debug due to the fact that compiler cannot check #define statements for error. So unless you do something like above it is just like #define except it is handled at compile time not by precomiler.

来源：https://stackoverflow.com/questions/50488831/use-of-constexpr-in-header-file

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c++11

constexpr