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: 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.

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

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.

• 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.