如何声明constexpr extern？

Question

是否可以声明一个变量extern constexpr并在另一个文件中定义它？

我试过了但是编译器报错：

Declaration of constexpr variable 'i' is not a definition

在 .h 中：

extern constexpr int i;

在 .cpp 中：

constexpr int i = 10;

Answer 1

没有。 Extern constexpr 没有任何意义。请阅读http://en.cppreference.com/w/cpp/language/constexpr

即位

it must be immediately constructed or assigned a value.

Answer 2

不，你不能这样做，标准是这样说的（第 7.1.5 节）：

1 The constexpr specifier shall be applied only to the definition of a variable or variable template, the declaration of a function or function template, or the declaration of a static data member of a literal type (3.9). If any declaration of a function, function template, or variable template has a constexpr specifier, then all its declarations shall contain the constexpr specifier. [Note: An explicit specialization can differ from the template declaration with respect to the constexpr specifier. Function parameters cannot be declared constexpr. — end note ]

标准给出的一些例子：

  constexpr void square(int &x);  // OK: declaration
  constexpr int bufsz = 1024;  // OK: definition
  constexpr struct pixel {  // error: pixel is a type
    int x;
    int y;
    constexpr pixel(int);  // OK: declaration
  };

  extern constexpr int memsz; // error: not a definition

Answer 3

是的，有点是...

//===================================================================
// afile.h

#ifndef AFILE
#define AFILE

#include <cstddef>
#include <iostream>

enum class IDs {

  id1,
  id2,
  id3,
  END

};

// This is the extern declaration of a **constexpr**, use simply **const**
extern const int ids[std::size_t(IDs::END)];

// These functions will demonstrate its usage

template<int id> void Foo() { std::cout << "I am " << id << std::endl; }

extern void Bar();

#endif // AFILE

//===================================================================
// afile.cpp

#include "afile.h"

// Here we define the consexpr. 
// It is **constexpr** in this unit and **const** in all other units
constexpr int ids[std::size_t(IDs::END)] = {

  int(IDs::id1),
  int(IDs::id2),
  int(IDs::id3)

};

// The Bar function demonstrates that ids is really constexpr
void Bar() {

  Foo<ids[0]      >();
  Foo<ids[1] + 123>();
  Foo<ids[2] / 2  >();

}

//===================================================================
// bfile.h

#ifndef BFILE
#define BFILE

// These functions will demonstrate usage of constexpr ids in an extern unit

extern void Baz();
extern void Qux();


#endif // BFILE

//===================================================================
// bfile.cpp

#include "afile.h"

// Baz demonstrates that ids is (or works as) an extern field
void Baz() {

  for (int i: ids) std::cout << i << ", ";
  std::cout << std::endl;

}

// Qux demonstrates that extern ids cannot work as constexpr, though
void Qux() {

#if 0 // changing me to non-0 gives you a compile-time error...

  Foo<ids[0]>();

#endif

  std::cout << "Qux: 'I don't see ids as consexpr, indeed.'" 
            << std::endl;

}

//===================================================================
// main.cpp

#include "afile.h"
#include "bfile.h"

int main(int , char **)
{

  Bar();
  Baz();
  Qux();

  return 0;
}

Answer 4

我同意上面的'swang'，但是有一个后果。考虑：

ExternHeader.hpp

extern int e; // Must be extern and defined in .cpp otherwise it is a duplicate symbol.

ExternHeader.cpp

#include "ExternHeader.hpp"
int e = 0;

ConstexprHeader.hpp

int constexpr c = 0; // Must be defined in header since constexpr must be initialized.

Include1.hpp

void print1();

Include1.cpp

#include "Include1.hpp"
#include "ExternHeader.hpp"
#include "ConstexprHeader.hpp"
#include <iostream>

void print1() {
    std::cout << "1: extern = " << &e << ", constexpr = " << &c << "\n";
}

Include2.hpp

void print2();

Include2.cpp

#include "Include2.hpp"
#include "ExternHeader.hpp"
#include "ConstexprHeader.hpp"
#include <iostream>

void print2() {
    std::cout << "2: extern = " << &e << ", constexpr = " << &c << "\n";
}

main.cpp

#include <iostream>
#include "Include1.hpp"
#include "Include2.hpp"

int main(int argc, const char * argv[]) {
    print1();
    print2();
    return 0;
}

打印：

1: extern = 0x1000020a8, constexpr = 0x100001ed0
2: extern = 0x1000020a8, constexpr = 0x100001ed4

IE constexpr 分配了两次，而 extern 分配了一次。这对我来说是违反直觉的，因为我 'expect' constexpr 比 extern.

更优化

编辑：const 和 constexpr 在分配方面具有相同的行为，因此从这个角度来看，行为符合预期。虽然，正如我所说，当我遇到 constexpr.

的行为时，我感到很惊讶

Answer 5

您可能需要的是 extern 和 constexpr 初始化，例如：

// in header
extern const int g_n;

// in cpp
constexpr int g_n = 2;

尽管在 Visual Studio 2017 年仅通过一致性模式提供支持：

Answer 6

C++17 inline 变量

这个很棒的 C++17 特性使我们能够：

方便地为每个常量使用一个内存地址
将其存储为 constexpr
从一个 header

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

不是main.cpp

#include "notmain.hpp"

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

编译并运行:

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.

C++ 标准保证地址相同。 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 说明如果没有给出static，那么它有外部链接。

另请参阅：How do inline variables work?

在 GCC 7.4.0、Ubuntu 18.04 中测试。

如何声明constexpr extern？

How to declare constexpr extern?

c++

extern

constexpr