Ternary operator, if I avoid writing ' expression 2 ' it works but if I not write 'expression 3 ' it gives an error

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Code

#include<iostream>

int main()
{
    int a=4,b,c;
    a==3 ? : b=a*a ;     // Works fine
    a==4 ? c=a*a : ;     // ERROR Expected Primary expression before ;
}

  • 1st conditional statement

    I did not write 'Expression 2' but it will not produce error

  • 2nd Conditional statement

    I did not write 'Expression 3' it gives an error

so why it is differentiating in 'Expression 2' and 'Expression 3' ?

conditional-operator c++
1 Answers

This is supported in GNU C/C++ and is called the Elvis operator. It's not allowed in standard C++ (e.g. MSVC++ doesn't support it).

It's described in the official documentation

6.8 Conditionals with Omitted Operands The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.

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