What is the difference between expressions : "int a" and "const int& a" as C++ function arguments?

Question

Let’s assume I have two functions:

int soma_a(int a, int b){
    return a + b;
}

and

int soma_b(const int& a, const int& b){
    return a + b;
}

What would be the difference between soma_a and soma_b

Answer 1

The first is being passed by value, that is, the value of the argument being used in the call of this function is copied to the function parameter soma_a().

The second argument value is not copied, a reference is used in place, i.e., an opaque pointer is created by pointing to the location where the argument value is and that address is copied to the function parameter soma_b(). How there’s a const means that the function is prohibited from changing this value, it will be read only, so there is a contract guaranteeing to the caller that the value will be intact.

In a int doesn’t make sense because the int It’s usually the same size or even smaller than the size of a pointer, so copying costs the same or more. usually this is more useful in cases where the past structure is larger than a pointer.

In written form both will behave exactly the same, but the former will be more efficient in this specific case.

If I didn’t have the const then it could be useful to make the parameter serve as output as well, then changing its value within the function would cause a side effect on the argument, after all the argument and parameter are in the same address, you passed the address, not a value independently, they are completely linked because it is the same thing. In this case the argument has to be a variable.

#include <iostream>
using namespace std;

int soma_b(int& a, const int& b) {
    a = 4;
    return a + b;
}

int main() {
    int x = 2;
    cout << soma_b(x, 3) << endl;
    cout << x;
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

important readings to better understand the subject and some related ones, even if in another language:

• What is the difference between parameter and argument?
• As is the C reference++?
• What is the difference between pointer and reference?
• Why in set methods in C++ I have to use the parameter as reference?
• Refection return in C++
• Memory allocation in C# - Value types and reference types

Answer 2

In C++ , in addition to variables, we can also use numbers or characters whose values do not change. They are called constants. In this case, in soma_b, you make reference to the variables a and b as constants, therefore, their values cannot be changed. (not even with a command cin >> a >> b >> endl;