在成员函数中的lambda捕获列表中使用成员变量 [英] Using member variable in lambda capture list inside a member function

问题描述

以下代码使用gcc 4.5.1编译，但不与VS2010 SP1编译：

  #include< iostream& 
 #include< vector> 
 #include< map> 
 #include< utility> 
 #include< set> 
 #include< algorithm> 
 
 using namespace std; 
 class puzzle 
 {
 vector< vector< int>>格; 
 map< int，set< int>>组; 
 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：

 错误C3480：'puzzle :: grid'：lambda捕获变量必须来自包含函数范围
警告C4573：'puzzle :: grid'需要编译器捕获this，但当前的默认捕获模式不允许它
  

/ p>

1>哪个编译器是正确的？

2>如何在VS2010中的lambda中使用成员变量？

解决方案

我相信VS2010这次是正确的，我会检查我是否有标准的方便， 't。

现在，它就像错误消息所说：你不能捕获lambda的封闭范围之外的东西。^† 不在封闭范围内，但此是（每次访问 grid 实际上发生在成员函数中的 this-> grid ）。对于你的usecase，捕获这工程，因为你马上就会使用它，你不想复制网格

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

然而，如果您想存储网格并将其复制以供稍后访问， c $ c> puzzle 对象可能已被销毁，您需要创建一个中间本地副本：

  vector< vector< int> > tmp（grid）; 
 auto lambda = [tmp]（）{}; //捕获每个副本的本地副本
  

---

'简化 - Google for达到范围或参见§5.1.2了解所有的血腥细节。

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;
}

This is the error:

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

So,

1> which compiler is right?

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

解决方案

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

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] << "\n"; }

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

---

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

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