C++ C++；创建临时对象以调用成员函数

下面代码中有编译错误的代码行（注释中）有什么问题？我认为它应该使用

const std:：string

调用

TestMe

构造函数，然后对其调用

operator（）

。但是它看起来像是在试图用变量

a

构造

TestMe

#include <iostream>

namespace
{
class TestMe
{
public:
    TestMe() {}
    TestMe( const std::string& me ) : me_( me ) {}

    bool operator()() const
    {
        std::cout << "calling operator()" << std::endl;
        return true;
    }

private:
    const std::string me_;
};
}


int main()
{
    const std::string a("a");

    //
    // construct an instance of TestMe and call its operator()
    //

    TestMe()();         // OK
    TestMe( "abc" )();  // OK
    TestMe tm( a );     // OK
    TestMe( a )();      // compile error: what's wrong with this?
    TestMe( { a } )();  // OK

    return 0;
}

“最烦人的解析”：

TestMe（a）（

被视为名为

a

的函数的声明，该函数不带任何参数并返回

TestMe

对象

稍微更改您的程序，使有问题的行不会与名称

a

冲突：

#include <iostream>

namespace
{
class TestMe
{
public:
    TestMe() {}
    TestMe( const std::string& me ) : me_( me ) {}

    bool operator()() const
    {
        std::cout << "calling operator()" << std::endl;
        return true;
    }

private:
    const std::string me_;
};
}


int main()
{
    const std::string a("a");

    //
    // construct an instance of TestMe and call its operator()
    //

    TestMe()();         // OK
    TestMe( "abc" )();  // OK
    TestMe tm( a );     // OK
    std::cout << "before most vexing parse\n";
    TestMe( b )();      // compiles, but has no effect
    std::cout << "after most vexing parse\n";
    TestMe( { a } )();  // OK

    return 0;
}

#include <iostream>

namespace
{
class TestMe
{
public:
    TestMe() {}
    TestMe( const std::string& me ) : me_( me ) {}

    bool operator()() const
    {
        std::cout << "calling operator()" << std::endl;
        return true;
    }

private:
    const std::string me_;
};
}


int main()
{
    const std::string a("a");

    //
    // construct an instance of TestMe and call its operator()
    //

    TestMe()();         // OK
    TestMe( "abc" )();  // OK
    TestMe tm( a );     // OK
    std::cout << "before most vexing parse\n";
    TestMe( b )();      // compiles, but has no effect
    std::cout << "after most vexing parse\n";
    TestMe( { a } )();  // OK

    return 0;
}

calling operator()
calling operator()
before most vexing parse
after most vexing parse
calling operator()