Chris Gregory

C++ Tutorial 7 - const

12/19/2016

Here we will go over the const keyword.

#include <cstdio>
#include <string>

using namespace std;

int main() {
    string message = "Message";
    message += " for you!";

    /* Visual Studio: */ getchar();
    return 0;
}

Here we declare a string and initialize it with the value "Message". We then append " for you!", making message contain "Message for you!". Let's extract out this appendage to a function.

void append(string& str) {
    str += " for you!";
}

    // in main()
    append(message);  // was `message += " for you!";`
          

When we pass a variable by reference to a function, we are sharing it with the function. When we share it, they are also given write access to the variable. Many times, we wish to share with only read access. Our append function requires write access into the variable because it outputs data into it. Let's make a print function to see why we would want read-only access.

#include <iostream>
void print(string& str) {
    cout << str;
}

The str parameter is taken by reference. Just like in append, we are able to modify the string. Since our print function doesn't actually modify the string, we should tell the computer that it won't. To do this, we add the keyword const before the ampersand marking it as a reference:

void print(string const& str) {
    cout << str;
}

Before it was const, writing print("The milk man is here"); would give us an error at compile time. Marking it as const allows it to work. This shows us an important aspect of const references: they allow the code to use temporary variables as arguments.

Since the parameter is taken as a const reference, it also prevents print() from modifying the string. So the following code would also not compile because it modifies the parameter that is marked as unchangeable:

void append(string const& str) {
    str += " for you!";
}

Note that const& parameters only mean that the access to the variable will be constant, not that the actual variable is const.