c++operatorsvariable-assignmentassignment-operatorcompound-assignment

Number raised to 2, 3, and 4

I have written the following simple C++ program to output the powers of a number input by the user to indices 2, 3, and 4:

int a = 0;

cin >> a;

int a2 = a * a;

cout << "\n" << a2 << "\n" << (a2 *= a) << "\n" << (a2 *= a) << "\n";

return 0;

The output for a = 2 was

16
16
16

Can anybody explain why that is?

Solution

  • Can anybody explain why that is?

    This is because you are using a C++ compiler that supports C++14, or earlier.

    Before C++17 the evaluation order for the following expression is not specified by the C++ standard:

    cout << "\n" << a2 << "\n" << (a2 *= a) << "\n" << (a2 *= a) << "\n";

    C++14, and earlier, allows the C++ compiler to implement any evaluation order for this expression. Before C++17 it was allowed (but not required), for both a2 *= a expressions to be evaluated first, setting a2 to 16, in the end, which gets couted three times.

    As a general rule of thumb: unless the evaluation order is specified it is entirely up to your compiler to evaluate the expression in any order. For example, with int x=foo() + bar(); either foo() or bar() may be called first, followed by the other one. The order of evaluation of +'s operands is not specified by the C++ standard. Your expression using << operators is the same thing.

    Before C++17 the above statement may produce different output with different C++ compilers, depending on which evaluation order they chose. It is entirely up to them.

    Starting with C++17 the evaluation order here is specified, and you will get the intuitively expected results.