c++ - a+b 和 operator+(a,b) 的区别

Question

考虑以下程序:

#include<functional>

typedef std::function< int( int ) > F;

F operator+( F, F )
{
    return F();
}

int f( int x ) { return x; }

int main()
{
    operator+(f,f); // ok
    f+f; // error: invalid operands to binary expression
}

为什么最后一行 f+f; 编译不通过？为什么它与 operator+(f,f); 不同？如果能引用该标准，我们将不胜感激。

Answer 1

f 的类型是内置类型。对内置类型对象的操作从不考虑用户定义的运算符。调用 operator+(f, f) 显式强制执行两次转换，除非强制执行否则不会发生。相关条款是 13.3.1.2 [over.match.oper] 第 1 段:

If no operand of an operator in an expression has a type that is a class or an enumeration, the operator is assumed to be a built-in operator and interpreted according to Clause 5. ...