C++ Evaluation order

Evaluation order defines in which order operands of an operator are evaluated. Before C++17, most operators had unspecified evaluation order, leading to undefined behavior in code like f(a++) + f(b++) where the order of side effects was unpredictable. C++17 specified the order for most operators, eliminating many pitfalls.

Don't write code that depends on evaluation order; prefer writing expressions with clear, sequential intent.

This example shows evaluation order issues and how to avoid them.

// compile: g++ -std=c++17 -o eval eval.cpp
// run: ./eval
// description: evaluation order pitfalls and safe patterns
 
#include <iostream>
 
int counter = 0;
 
int inc() {
    std::cout << "inc called\n";
    return ++counter;
}
 
int main() {
    counter = 0;
 
    // In C++17+, left-to-right order for assignment
    int a = inc() + inc();  // evaluates left to right
 
    // Avoid unclear orders: use separate statements
    int x = inc();
    int y = inc();
 
    std::cout << "counter = " << counter << "\n";
    std::cout << "a = " << a << ", x = " << x << ", y = " << y << "\n";
 
    return 0;
}