Why in conditional operator (?:), second and third operands must have the same type?

Question

Why in conditional operator(?:), second and third operands must have the same type?

My code like this:

#include 

        

Answer 1

You should try to decompose what's happening to understand:

cout << ( a>b ? "a is greater\n" : b );

This translates to :

operator<<(cout, ( a>b ? "a is greater\n" : b ));

At that stage the compiler must choose one of the following overloads:

ostream& operator<<(ostream&, int);
ostream& operator<<(ostream&, const char*);

But it can't because the type of result of the ternary operator is not known yet (only at runtime).

To make things clearer think like this:

 auto c = a>b ? "test" : 0;

What would be the type of c ? It can't be decided at compile time. C++ is a statically typed language. All types must be known at compile time.

EDIT:

You are thinking of a ? b : c in the following way:

if (a)
    b;
else
    c;

While it is actually this:

if (a)
    return b;
else
    return c;