c++ - VC++ 2013 : using-declaration + redefinition of member function leads to compile error

Question

我想通过指定策略允许修改我的类的行为。该策略应该用作 boost::variant 的访问者。有适合大多数情况的默认策略，但用户可能需要添加或替换一些重载。

我发现 vc++ 2013 没有编译此代码并出现错误 C3066: There are multiple ways that an object of this type of can be called with these arguments。相同的代码在 gcc 和 clang 中按预期编译和工作。

是 vc++ 2013 的错误吗？

#include <iostream>

struct DefaultPolicy
{
    void operator()( bool ) { std::cout << "Base: bool" << std::endl; }
    void operator()( int ) { std::cout << "Base: int" << std::endl; }
};

struct UserModifiedPolicy : public DefaultPolicy
{
    using DefaultPolicy::operator();
    void operator()( int ) { std::cout << "Derived: int" << std::endl; }
    void operator()( float ) { std::cout << "Derived: float" << std::endl; }
};

int main()
{
    UserModifiedPolicy()(true);
    UserModifiedPolicy()(1); // <-- ERROR HERE
    UserModifiedPolicy()(1.f);
    return 0;
}

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UPD 这个例子适用于 vc++ 2010。看起来它是 2013 版本中的一个错误。

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UPD 解决方法

#include <iostream>

struct DefaultPolicy
{
    void operator()( bool ) { std::cout << "Base: bool" << std::endl; }
    void operator()( int ) { std::cout << "Base: int" << std::endl; }
};

struct UserModifiedPolicy : public DefaultPolicy
{
    // Using template to forward a call to the base class:
    template< class T >
    void operator()( T && t ) { DefaultPolicy::operator()( std::forward<T>(t) ); }

    void operator()( int ) { std::cout << "Derived: int" << std::endl; }
    void operator()( float ) { std::cout << "Derived: float" << std::endl; }
};

int main()
{
    UserModifiedPolicy()(true);
    UserModifiedPolicy()(1);
    UserModifiedPolicy()(1.f);
    return 0;
}

Answer 1

代码格式正确。 7.3.3/15:

When a using-declaration brings names from a base class into a derived class scope, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name, parameter-type-list (8.3.5), cv-qualification, and ref-qualifier (if any) in a base class (rather than conflicting).

所以 UserModifiedPolicy::operator()(int) 应该仍然隐藏 DefaultPolicy::operator()(int)。 operator() 的名称查找应该找到三个成员 DefaultPolicy::operator()(bool)、UserModifiedPolicy::operator()(int) 和 UserModifiedPolicy::operator()(float)。