Why doesn't this compile?

Question

When I try to declare iss using the first form, g++ gives me "error: no match for 'operator>>' in 'iss >> s'". But don't the two different declarations do the same thing?

#include <iostream>
#include <sstream>
#include <string>


int main() {
    const char *buf = "hello world";
    std::string ss(buf);
    //std::istringstream iss(std::string(buf)); // this doesn't work
    std::istringstream iss(ss); // but this does
    std::string s;
    iss >> s;
}

Answer 1

This is known as the "most vexing parse" of C++: what looks like an instance declaration to you actually looks like a function declaration to the compiler.

std::string name(); //function declaration
std::string name;  //object declaration with default constructor

std::stringstream ss(std::string(buf));  //function declaration
std::stringstream ss(std::string buf);  //another function declaration
std::stringstream ss(std::string);  //also a function declaration
std::stringstream ss(std::string());  //ditto, argument names are optional

std::stringstream ss((std::string(buf))); //object declaration

Note the extra brackets in the last example. These brackets wouldn't be legal in a function declaration.

The first example with the default constructor is well-known. What adds unclarity in the second case is that brackets around parameter names in C++ are legal but optional. For example, you can define a function like this:

void foo(int (bar))
{}

Basically you'll run into this every time when all arguments to a constructor are temporaries from constructor invocations that take 0 or 1 arguments, and the quick solution is to put extra brackets around one of the arguments.