# C++ Evaluation order

**[Evaluation order](https://en.cppreference.com/w/cpp/language/eval_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.

## Example

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

```cpp
// 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;
}
```