在成员函数内的 lambda 捕获列表中使用成员变量

问题描述

以下代码使用 gcc 4.5.1 编译但不适用于 VS2010 SP1:

The following code compiles with gcc 4.5.1 but not with VS2010 SP1:

#include <iostream>
#include <vector>
#include <map>
#include <utility>
#include <set>
#include <algorithm>

using namespace std;
class puzzle
{
        vector<vector<int>> grid;
        map<int,set<int>> groups;
public:
        int member_function();
};

int puzzle::member_function()
{
        int i;
        for_each(groups.cbegin(),groups.cend(),[grid,&i](pair<int,set<int>> group){
                i++;
                cout<<i<<endl;
        });
}
int main()
{
        return 0;
}

这是错误:

error C3480: 'puzzle::grid': a lambda capture variable must be from an enclosing function scope
warning C4573: the usage of 'puzzle::grid' requires the compiler to capture 'this' but the current default capture mode does not allow it

所以，

1> 哪个编译器是对的?

1> which compiler is right?

2> 如何在 VS2010 的 lambda 中使用成员变量?

2> How can I use member variables inside a lambda in VS2010?

推荐答案

我相信 VS2010 这次是正确的，我会检查我是否有方便的标准，但目前我没有.

I believe VS2010 to be right this time, and I'd check if I had the standard handy, but currently I don't.

现在，就像错误消息所说的那样:您无法捕获 lambda 封闭范围之外的内容.^† grid 不在封闭范围内，但是 this 是(在成员函数中，对 grid 的每次访问实际上都是作为 this->grid 发生的).对于您的用例，捕获 this 有效，因为您将立即使用它并且您不想复制 grid

Now, it's exactly like the error message says: You can't capture stuff outside of the enclosing scope of the lambda.^† grid is not in the enclosing scope, but this is (every access to grid actually happens as this->grid in member functions). For your usecase, capturing this works, since you'll use it right away and you don't want to copy the grid

auto lambda = [this](){ std::cout << grid[0][0] << "
"; }

但是，如果您想要存储网格并复制它以供以后访问，而您的 puzzle 对象可能已经被销毁，则您需要制作一个中间的本地副本:

If however, you want to store the grid and copy it for later access, where your puzzle object might already be destroyed, you'll need to make an intermediate, local copy:

vector<vector<int> > tmp(grid);
auto lambda = [tmp](){}; // capture the local copy per copy

<小时>

† 我正在简化 - 谷歌搜索达到范围"或参阅第 5.1.2 节了解所有血腥细节.

† I'm simplifying - Google for "reaching scope" or see §5.1.2 for all the gory details.

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