C ++嵌套lambda bug在VS2010与lambda参数捕获？ [英] C++ nested lambda bug in VS2010 with lambda parameter capture?

问题描述

我使用Visual Studio 2010，它显然在lambdas有一些错误的行为，并有这个嵌套的lambda，其中内部lambda返回第二个lambda包装为std ::函数（cf. MSDN上的高阶Lambda函数）：

I'm using Visual Studio 2010, which apparently has some buggy behavior on lambdas, and have this nested lambda, where the inner lambda returns a second lambda wrapped as a std::function (cf. "Higher-order Lambda Functions" on MSDN):

int x = 0;
auto lambda = [&]( int n ) 
{ 
    return std::function<void()>( 
        [&] // Note capture
        { 
            x = n; 
        } 
    ); 
};

lambda( -10 )(); // Call outer and inner lambdas

assert( -10 == x ); // Fails!

这会进行编译，但在断言时失败。具体来说，内部lambda中的n未初始化（0xCCCCCCCC），但x已成功修改为其值。如果我将内部lambda的捕获子句改为[&，n]，则assert如预期那样传递。这是VS2010的一个错误还是我不明白lambda捕获如何工作？

This compiles but fails at the assert. Specifically, n in the inner lambda is uninitialized (0xCCCCCCCC), but x is successfully modified to its value. If I change the inner lambda's capture clause to "[&,n]", the assert passes as expected. Is this a bug with VS2010 or have I not understood how lambda capture works?

推荐答案

这不是一个错误，因为 n 在lambdas return语句之后超出作用域，因此通过引用获取的捕获将在您使用它时失效。

It is not a bug, since n goes out of scope after lambdas return statement, thus the capture by reference is invalidated by the time you use it.

int x = 0;
auto lambda = [&]( int n ) 
{ 
    return std::function<void()>( // n is local to "lambda" and is destroyed after return statement, thus when you call the std::function, the reference capture of n is invalid.
        [&]
        { 
            x = n; // Undefined behaviour
        } 
    ); 
};

auto tmp = lambda(-10); 
// n is no longer valid
tmp(); // calling tmp which uses reference of n which is alrdy destroyed.

assert( -10 == x ); // Fails!

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