c++ - Visual C++ 2015 std::function与C2893

因此，我正在试验std :: function和模板，并在下面提出了代码。不幸的是，事实证明它导致Visual Studio 2015出现C2893（也许在其他编译器上可以正常工作吗？）。我不知道这是否有帮助，但是当我用void *替换NextHandler的第一个参数时，编译错误就消失了...

#include <functional>

template<typename T>
class Block
{
public:

    typedef Block CurrentBlock;
    typedef T NextBlock;

    typedef std::function<void (CurrentBlock& currentBlock,
        NextBlock& nextBlock)>
        NextHandler;

private:

    NextBlock*  _nextBlock;
    NextHandler _nextHandler;

protected:

    virtual void
    doProcess() = 0;

public:

    NextBlock&
    next(NextBlock& nextBlock, NextHandler handler)
    {
        _nextBlock = &nextBlock;
        _nextHandler = handler;
        return nextBlock;
    }

    void
    process()
    {
        doProcess();
        _nextHandler(*this, *_nextBlock);
        _nextBlock->process();
    }
};

template<>
class Block<void>
{
protected:

    virtual void
    doProcess() = 0;

public:

    void
    process()
    {
        doProcess();
    }
};

class MockBlock1 : public Block<void>
{
public:

    void
    doProcess() override
    {
    }
};

class MockBlock0 : public Block<MockBlock1>
{
public:

    void
    doProcess() override
    {
    }
};

void someFunction()
{
    MockBlock0 b0;
    MockBlock1 b1;

    b0.next(b1, [](MockBlock0& b0, MockBlock1& b1)
        {
        });
}

完整的错误消息是：

c:\program files (x86)\microsoft visual studio 14.0\vc\include\type_traits(1494): error C2893: Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Types &&...)'
c:\program files (x86)\microsoft visual studio 14.0\vc\include\type_traits(1494): note: With the following template arguments:
c:\program files (x86)\microsoft visual studio 14.0\vc\include\type_traits(1494): note: '_Callable=_Decayed &'
c:\program files (x86)\microsoft visual studio 14.0\vc\include\type_traits(1494): note: '_Types={Block<MockBlock1> &, MockBlock1 &}'
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(210): note: see reference to function template instantiation 'void std::_Invoke_ret<_Rx,_Callable&,Block<MockBlock1>&,MockBlock1&>(std::_Forced<_Rx,true>,_Callable &,Block<MockBlock1> &,MockBlock1 &)' being compiled
        with
        [
            _Rx=void,
            _Callable=_Decayed
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(208): note: while compiling class template member function 'void std::_Func_impl<_Decayed,_Alloc,_Ret,Block<MockBlock1> &,MockBlock1 &>::_Do_call(Block<MockBlock1> &,MockBlock1 &)'
        with
        [
            _Alloc=std::allocator<int>,
            _Ret=void
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(136): note: see reference to class template instantiation 'std::_Func_impl<_Decayed,_Alloc,_Ret,Block<MockBlock1> &,MockBlock1 &>' being compiled
        with
        [
            _Alloc=std::allocator<int>,
            _Ret=void
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(339): note: see reference to class template instantiation 'std::_Is_large<_Myimpl>' being compiled
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(318): note: see reference to function template instantiation 'void std::_Func_class<_Ret,Block<MockBlock1> &,MockBlock1 &>::_Reset_alloc<_Ty,std::allocator<int>>(_Fx &&,const _Alloc &)' being compiled
        with
        [
            _Ret=void,
            _Ty=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>,
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>,
            _Alloc=std::allocator<int>
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(318): note: see reference to function template instantiation 'void std::_Func_class<_Ret,Block<MockBlock1> &,MockBlock1 &>::_Reset_alloc<_Ty,std::allocator<int>>(_Fx &&,const _Alloc &)' being compiled
        with
        [
            _Ret=void,
            _Ty=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>,
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>,
            _Alloc=std::allocator<int>
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(484): note: see reference to function template instantiation 'void std::_Func_class<_Ret,Block<MockBlock1> &,MockBlock1 &>::_Reset<someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>>(_Fx &&)' being compiled
        with
        [
            _Ret=void,
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>
        ]
c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(484): note: see reference to function template instantiation 'void std::_Func_class<_Ret,Block<MockBlock1> &,MockBlock1 &>::_Reset<someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>>(_Fx &&)' being compiled
        with
        [
            _Ret=void,
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>
        ]
c:\test\blocks.cpp(88): note: see reference to function template instantiation 'std::function<void (Block<MockBlock1> &,MockBlock1 &)>::function<someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>>(_Fx)' being compiled
        with
        [
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>
        ]
c:\test\blocks.cpp(88): note: see reference to function template instantiation 'std::function<void (Block<MockBlock1> &,MockBlock1 &)>::function<someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>>(_Fx)' being compiled
        with
        [
            _Fx=someFunction::<lambda_3b7d9d95683ccbaabd52b19687773e09>
        ]

最佳答案

什么是MockBlock0::NextHandler aka Block<MockBlock1>::NextHandler？

是std::function<void (Block<MockBlock1>&, MockBlock1&)>。

可以用[](MockBlock0& b0, MockBlock1& b1) { /* stuff */ }类型的任何左值调用Block<MockBlock1>吗？不能，因为并非所有Block<MockBlock1>都是MockBlock0。

因此，错误。

关于c++ - Visual C++ 2015 std::function与C2893，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/34820607/