如何修复变量声明和全局构造函数警告？

Question

我正在阅读 C++ 如何编程 并尝试使用我目前所学的知识（哈！）并编写了这个程序。 （这是我自己的事情，不是书中的练习。）输出是我想要的，但我无法修复警告。

我在 Game.h 中对 random 的使用是基于我在书中看到的。

如果我将导致警告的行放在main() 中，错误就会消失，但编译器会抛出一个致命错误，因为它不再可以访问变量engine。我明白了。

我感到很沮丧，因为我以为我在学习 C++，但 C++11 的东西似乎很快就让我忘记了。

游戏.h

#include <iostream>
#include <array>
#include <ctime>
#include <random>

std::default_random_engine engine( static_cast< unsigned int >( time(0) ) );
std::uniform_int_distribution< int > randomInt( 0, 23 );
void initPieces( std::array< int, 24 >& );
void showPieces( std::array< int, 24 > );

main.cpp

#include "Game.h"

int main() {
  std::array< int, 24 > piecesPlayer1 = {};
  std::array< int, 24 > piecesPlayer2 = {};
  initPieces ( piecesPlayer1 );
  initPieces ( piecesPlayer2 );
}

void initPieces( std::array< int, 24 >& myPieces) {
  for ( unsigned int i = 0; i < 24; i += 3 ) {
    myPieces[ i ] = 1;
    myPieces[ i + 1 ] = 2;
    myPieces[ i + 2 ] = 3;
  }
  for ( unsigned int i = 0; i < 24; i++ ) {
    int s = randomInt( engine );
    std::swap ( myPieces[ i ], myPieces[ static_cast<unsigned int>( s ) ] );
  }
  showPieces ( myPieces );
}

void showPieces( std::array< int, 24 > myPieces) {
  for ( unsigned int i = 0; i < 24; i++ ) {
    std::cout << myPieces[ i ] << " ";
  }
  std::cout << std::endl;
}

编辑：我忘了包括警告。

rm -fr build/*
clang++ -std=c++11 -stdlib=libc++ -Weverything -Wno-c++98-compat src/main.cpp -o build/main -Isrc/
In file included from src/main.cpp:1:
src/Game.h:7:28: warning: no previous extern declaration for non-static variable 'engine' [-Wmissing-variable-declarations]
std::default_random_engine engine( static_cast< unsigned int >( time(0) ) );
                           ^
src/Game.h:7:28: warning: declaration requires a global constructor [-Wglobal-constructors]
std::default_random_engine engine( static_cast< unsigned int >( time(0) ) );
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/Game.h:8:38: warning: no previous extern declaration for non-static variable 'randomInt' [-Wmissing-variable-declarations]
std::uniform_int_distribution< int > randomInt( 0, 23 );
                                     ^
src/Game.h:8:38: warning: declaration requires a global constructor [-Wglobal-constructors]
std::uniform_int_distribution< int > randomInt( 0, 23 );
                                     ^~~~~~~~~~~~~~~~~~
4 warnings generated.
./build/main
2 3 2 3 3 2 2 2 2 1 1 3 2 1 3 1 1 1 3 3 3 1 1 2
2 3 3 1 2 3 3 2 1 2 1 2 1 1 3 2 3 1 3 2 3 1 2 1

Answer 1

使用C++17，你可以用inline装饰game.h：

#pragma once

#include <iostream>
#include <array>
#include <ctime>
#include <random>

inline std::default_random_engine engine(static_cast< unsigned int >(time(0)));
inline std::uniform_int_distribution< int > randomInt(0, 23);
void initPieces(std::array< int, 24 >&);
void showPieces(std::array< int, 24 >);

对于旧的 C++，将标头更改为：

#pragma once

#include <iostream>
#include <array>
#include <ctime>
#include <random>

extern std::default_random_engine engine;
extern std::uniform_int_distribution< int > randomInt;
void initPieces(std::array< int, 24 >&);
void showPieces(std::array< int, 24 >);

将定义放入 main.cpp 或其他一些编译单元，例如游戏.cpp.

std::default_random_engine engine(static_cast< unsigned int >(time(0)));
std::uniform_int_distribution< int > randomInt(0, 23);