问题:

Can somebody tell me what precisely

operator std::string()

stands for?

回答1:

It is a conversion operator that allows the object to be explicitly or implicitly casted to std::string. When such a cast occurs, the operator is invoked and the result of the cast is the result of the invocation.

As an example of an implicit cast, suppose you had a function that accepted type std::string or const std::string&, but not the given object type. Passing your object to that function would result in the conversion operator being invoked, with the result passed to the function instead of your type.

回答2:

It is a cast operator. Any class that defines this type can be used anywhere a std::string is required. For instance,

class Foo {
public:
    operator std::string() const { return "I am a foo!"; }
};
...
Foo foo;
std::cout << foo; // Will print "I am a foo!".

Cast operators are almost always a bad idea, since there is invariably a better way to achieve the same result. In the above case, you are better off defining operator<<(std::ostream&, const Foo&).