Const correctness and API design | C++

const means an object will not be modified through this name or interface. In C++, const is a design tool, not decoration.

Const variables

Use const for values that should not change:

const int max_retries{3};

This prevents accidental reassignment and tells the reader the value is stable.

Const references

Use const& to pass large read-only objects:

void print_report(const Report& report);

The function avoids copying and promises not to mutate the report.

Const member functions

A member function marked const promises not to modify the object:

class Account {
public:
    double balance() const {
        return balance_;
    }

    void deposit(double amount) {
        balance_ += amount;
    }

private:
    double balance_{0.0};
};

balance() can be called on a const Account. deposit() cannot.

Const enables use

If a function accepts a const object, it can only call const member functions:

void print_balance(const Account& account) {
    std::cout << account.balance() << "\n";
}

Forgetting const on read-only member functions makes useful code harder to write.

Const is not ownership

const does not mean an object lives longer. It does not mean thread-safe. It does not mean immutable globally.

It means this access path cannot modify the object.

const Account& view = account;

Other non-const references to account might still mutate it.

What to carry forward

const communicates and enforces non-mutation through an interface
use const variables for stable values
use const& for large read-only parameters
mark read-only member functions const
const correctness makes APIs easier to use safely
const does not solve lifetime or ownership

Next, you will write reusable code with templates.