Question

Compiling with clang++ 4.1:

class A
{
public:
    A(const char *s=0) : _s(s) {}
    const char *_s;
};

void f(A a)
{
    cout << a._s << endl;
}

int main()
{
    f("test");
    return 0;
}

prints,

test

whereas if I define f as follows,

void fA(A &a)
{
    cout << a._s << endl;
}

I get a compilation error,

clang++ -std=c++11 -stdlib=libc++ -o test test.cpp
test.cpp:14:5: error: no matching function for call to 'f'
    f("9000");
    ^
test.cpp:7:6: note: candidate function not viable: no known conversion from
      'const char [5]' to 'A &' for 1st argument;
void f(A &a)
     ^

Why? I don’t understand why making f take a reference leads to problems.

Answer 1

f("test"); attempts to create a temporary A – f(A("test"));, but that temporary can’t bind to a non-const reference. The pass-by-value is okay because a copy is theoretically made (in practice, copy elision will occur).