Why do std::istringstream only work here with a constructor?

Question

The code below compiles and works as expected. On line 11, when I change
std::istringstream iss(temp)
to
std::istringstream iss()
, it stops working. An error occurs on line 18, iss.str(temp):
expression must have class type but it has type "std::istringstream (*)()"

Why would a change to the constructor make a difference? I checked the docs, it shouldn't make a difference, but I must be missing something. Any thoughts?

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

int main() {

    int n, m;
    std::string temp;
    std::vector<int> ranked, player;
    std::istringstream iss();

    // Get the Input

    std::getline(std::cin, temp);
    n = std::stoi(temp);
    std::getline(std::cin, temp);
    iss.str(temp);
    while(!iss.eof()) {
        std::getline(iss, temp, ' ');
        ranked.push_back(std::stoi(temp));
    }

    return 0;
}

Look up "Most Vexing Parse" - if an expression *can* be interpreted as a function declaration, then it *will* be so interpreted. — Adrian Mole, Jun 25 '21 at 14:26

score 3 · Answer 1 · answered Jun 25 '21 at 14:27

3

std::istringstream iss();

is a declaration of a function iss. Remove the () to declare an object.

Also note that your usage of while(!iss.eof()) is wrong. The loop should be:

while(std::getline(iss, temp, ' ')) {
    ranked.push_back(std::stoi(temp));
}

answered Jun 25 '21 at 14:27

MikeCAT

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I saw something like the loop you mentioned in other posts, but I looked at the documentation (https://www.cplusplus.com/reference/string/string/getline/?kw=getline) and the return value didn't say anything about a boolean, so I'm a little confused. – BrianZhang Jun 25 '21 at 14:59

score 2 · Answer 2 · edited Jun 25 '21 at 14:42

2

std::istringstream iss() is the declaration of function with name iss and return value std::istringstream.

edited Jun 25 '21 at 14:42

Evg

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answered Jun 25 '21 at 14:26

Evgeny

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Why do std::istringstream only work here with a constructor?

2 Answers2