6.6 — Internal linkage

In lesson 6.3 -- Local variables, we said, “An identifier’s linkage determines whether other declarations of that name refer to the same object or not”, and we discussed how local variables have no linkage.

Global variable and functions identifiers can have either internal linkage or external linkage. We’ll cover the internal linkage case in this lesson, and the external linkage case in lesson 6.7 -- External linkage and variable forward declarations.

An identifier with internal linkage can be seen and used within a single translation unit, but it is not accessible from other translation units (that is, it is not exposed to the linker). This means that if two source files have identically named identifiers with internal linkage, those identifiers will be treated as independent (and not result in an ODR violation for having duplicate definitions).

Global variables with internal linkage

Global variables with internal linkage are sometimes called internal variables.

To make a non-constant global variable internal, we use the static keyword.

#include <iostream>

static int g_x{}; // non-constant globals have external linkage by default, but can be given internal linkage via the static keyword

const int g_y{ 1 }; // const globals have internal linkage by default
constexpr int g_z{ 2 }; // constexpr globals have internal linkage by default

int main()
{
    std::cout << g_x << ' ' << g_y << ' ' << g_z << '\n';
    return 0;
}

Const and constexpr global variables have internal linkage by default (and thus don’t need the static keyword -- if it is used, it will be ignored).

Here’s an example of multiple files using internal variables:

a.cpp:

[[maybe_unused]] constexpr int g_x { 2 }; // this internal g_x is only accessible within a.cpp

main.cpp:

#include <iostream>

static int g_x { 3 }; // this separate internal g_x is only accessible within main.cpp

int main()
{
    std::cout << g_x << '\n'; // uses main.cpp's g_x, prints 3

    return 0;
}

This program prints:

3

Because g_x is internal to each file, main.cpp has no idea that a.cpp also has a variable named g_x (and vice versa).

The one-definition rule and internal linkage

In lesson 2.7 -- Forward declarations and definitions, we noted that the one-definition rule says that an object or function can’t have more than one definition, either within a file or a program.

However, it’s worth noting that internal objects (and functions) that are defined in different files are considered to be independent entities (even if their names and types are identical), so there is no violation of the one-definition rule. Each internal object only has one definition.

Functions with internal linkage

Because linkage is a property of an identifier (not of a variable), function identifiers have the same linkage property that variable identifiers do. Functions default to external linkage (which we’ll cover in the next lesson), but can be set to internal linkage via the static keyword:

add.cpp:

// This function is declared as static, and can now be used only within this file
// Attempts to access it from another file via a function forward declaration will fail
[[maybe_unused]] static int add(int x, int y)
{
    return x + y;
}

main.cpp:

#include <iostream>

int add(int x, int y); // forward declaration for function add

int main()
{
    std::cout << add(3, 4) << '\n';

    return 0;
}

This program won’t link, because function add is not accessible outside of add.cpp.

Quick Summary

// Internal global variables definitions:
static int g_x;          // defines non-initialized internal global variable (zero initialized by default)
static int g_x{ 1 };     // defines initialized internal global variable

const int g_y { 2 };     // defines initialized internal global const variable
constexpr int g_y { 3 }; // defines initialized internal global constexpr variable

// Internal function definitions:
static int foo() {};     // defines internal function

We provide a comprehensive summary in lesson 6.11 -- Scope, duration, and linkage summary.