Linkage of Constants

In C++, const and constexpr variables have internal linkage by default to ensure that their values remain consistent across translation units and to avoid potential linking conflicts.

Internal Linkage

• Internal Linkage: By default, const and constexpr variables are only accessible within the file they are defined in.
• Reasoning: This prevents different parts of the program from inadvertently modifying these variables and ensures their values are consistent wherever used.

Here’s an example:

1// constants.cpp
2const int MaxUsers{100};
3constexpr float Pi{3.14159f};

Explanation

• Consistency: Having internal linkage ensures that MaxUsers and Pi are consistent within constants.cpp and can't be altered by other files.
• Avoiding Conflicts: Internal linkage prevents multiple definition errors that could arise if these variables were defined in multiple files.

Changing Linkage

You can give const and constexpr variables external linkage using the extern keyword:

1// constants.h
2#pragma once
3
4extern const int MaxUsers;
5extern constexpr float Pi;

1// constants.cpp
2#include "constants.h"
3
4const int MaxUsers{100};
5constexpr float Pi{3.14159f};

Below, we access and use these variables:

1// main.cpp
2#include <iostream>
3#include "constants.h"
4
5int main() {
6  std::cout << "MaxUsers: " << MaxUsers << '\n';
7  std::cout << "Pi: " << Pi << '\n';
8}

1g++ main.cpp constants.cpp -o myProgram
2./myProgram

1MaxUsers: 100
2Pi: 3.14159

Benefits of Internal Linkage

1. Encapsulation: Keeps the variable's scope limited to the defining file, promoting encapsulation and modularity.
2. Avoids Multiple Definitions: Prevents multiple definition errors and ensures a single definition per translation unit.
3. Consistency: Ensures the value of const and constexpr variables remains consistent across the file, avoiding accidental changes from other parts of the program.

Summary

• Default Behavior: By default, const and constexpr variables have internal linkage, making them file-local.
• Purpose: This design choice avoids linking conflicts and ensures value consistency.
• Modifying Linkage: If needed, their linkage can be changed to external using the extern keyword.

Understanding the default internal linkage of const and constexpr variables helps in writing clear and conflict-free code, ensuring variables are used as intended without unintended side effects.