头文件中的Lambda错误 [英] Lambda in header file error

问题描述

在我的一个课程中，我试图将std::priority queue与指定的lambda进行比较:

In one of my classes, I am trying to use std::priority queue with a specified lambda for comparison:

#pragma once
#include <queue>
#include <vector>

auto compare = [] (const int &a, const int &b) { return a > b; };
class foo
{
public:
    foo() {  };
    ~foo() {  };
    int bar();
private:
    std::priority_queue< int, std::vector<int>, decltype(compare)> pq;
};

我的程序可以完美地编译，直到我添加一个.cpp文件来附加标题:

My program compiles perfectly until I add a .cpp file to accompany the header:

#include "foo.h"

int foo::bar()
{
    return 0;
}

这一次，我的编译器生成了一个错误:

This time, my compiler generates an error:

>main.obj : error LNK2005: "class <lambda> compare" (?compare@@3V<lambda>@@A) already defined in foo.obj

如果我的头文件包含lambda，为什么不能创建随附的.cpp文件?

Why can't I create a accompanying .cpp file if my header file contains a lambda?

编译器:Visual Studio 2012

Compiler: Visual Studio 2012

我的main.cpp:

#include "foo.h"

int main(){
    return 0;
}

推荐答案

按照@Rapptz的建议，

As @Rapptz suggested,

const auto compare = [] (const int &a, const int &b) { return a > b; };

解决了问题.为什么?

内部与外部链接.默认情况下，与int一样，auto具有外部链接.因此:

Internal vs External linkage. By default, auto, like int has external linkage. So just how:

int j = 5;

在foo.h中随后会被foo.cpp包含的

会抛出

In foo.h that would later be included by foo.cpp throws a

错误2错误LNK2005:Header.obj中已经定义了"int j"(?j @@ 3HA)

Error 2 error LNK2005: "int j" (?j@@3HA) already defined in Header.obj

(VS 2013)

但是，const默认将链接设置为内部，这意味着该链接只能在一个翻译单元中访问，从而避免了该问题.

However, const makes the linkage internal by default, which means it is only accessible in one translation unit, thereby avoiding the problem.

这篇关于头文件中的Lambda错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！