CODEWITHSUNDEEP
c++reference
city
city

Reputation: 2144

Why it does not work when I pass reference to this function?

The code below:

void test(string &s){ // if the argument is "string s", it works
    return test(s+',');
}

The compiler reports cannot find the function: test(std::basic_string).

I think the compiler would create a temporary string (== s+','), and I can pass its reference. But it seems I am wrong. I do not know why I cannot pass the reference of this temporary string.

Upvotes: 2

Views: 914

Answers (4)

Mike Seymour
Mike Seymour

Reputation: 254431

You can't bind a temporary to a non-constant reference. You could either take the argument by const reference (or, as you point out, by value)

void test(string const & s){ // or string s
    return test(s+',');
}

or use a named variable rather than a temporary

void test(string & s){
    std::string s2 = s + ',';
    return test(s2);
}

As noted, at great length, in the comments, this code has undefined runtime behaviour and shouldn't be used in "real" code; it's purpose is just a minimal example of how to fix the observed compilation error

Upvotes: 2

brunocodutra
brunocodutra

Reputation: 2349

Standard C++ does not allow a non-const lvalue reference to be bound to an rvalue. In your example, the result of the expression s+',' is a temporary and thus an rvalue, so the compiler is required to discard the overload of test expecting an lvalue reference, leaving it no overload available to call. That's why it complains about not being able to find the function test.

To solve this issue you have to provide an overload whose parameter may be bound to an rvalue. As you realized yourself, expecting an argument by-copy works, but it may imply unnecessary overhead by calling copy/move-ctors. A better option would be to expect the argument by reference. Since const lvalue references may be bound to rvalues, declaring the function as follows solves the problem.

void test(std::string const& s)

Upvotes: 0

Arun Sharma
Arun Sharma

Reputation: 517

But first you should have seen this question String Concatenation

concatenating strings using "+" in c++

Alternative Solution

void test(string  &s)
{
    s.append(",");
    return test(s);
}

Upvotes: 0

marcinj
marcinj

Reputation: 49976

make it const:

void test(const std::string &s){ // if the argument is "string s", it works
    return test(s+',');
}

Upvotes: 2

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