In C++ when should you pass by reference or by value?

You should pass by reference when you are going to change a variable and by value when you are just going to read or use it in some sort of calculation. It's that simple For example if you have a sum(int , int ) function you can do a cout << sum(a,b); but you can also do a myvar=sum(a,b); You certainly don't have to send the sum back as a separate variable.

You pass by value if the value is not to be modified by the function and the value itself may be calculated. So you would tend to pass by value the numbers that go into mathematical functions, for example sqrt(a*a+b*b). You pass by reference when the value is an output or both an input and output from a function, or when the value is very large (in which case a const reference will be used). For example a function that orders two parameters would necessarily need to have them passed by reference. It is not true that classes have to be passed by reference, or though it is normal. Everything is a class, so the fact you can pass somethings by value kind of proves the point. However you do need things like copy constructors to pass by value and when classes get large passing them by value both involves much effort to copy them and much stack space to store them.

You should pass by reference when you are going to change a variable and by value when you are just going to read or use it in some sort of calculation. It's that simple For example if you have a sum(int , int ) function you can do a cout << sum(a,b); but you can also do a myvar=sum(a,b); You certainly don't have to send the sum back as a separate variable.