---
title: "Program and Linkage  (C++) | Microsoft Docs"
ms.custom: ""
ms.date: "04/09/2018"
ms.technology: ["cpp-language"]
ms.topic: "language-reference"
dev_langs: ["C++"]
ms.assetid: a6493ba0-24e2-4c89-956e-9da1dea660cb
author: "mikeblome"
ms.author: "mblome"
ms.workload: ["cplusplus"]
---
# Program and Linkage  (C++)

In a C++ program, each global symbol can be defined only once, even if the program consists of multiple translation units. A translation unit consists of an implementation file (.cpp, .hpp, .cxx, etc) and all the headers that it includes directly or indirectly. A program consists of one or more translation units linked together. 

## Linkage vs. scope

The concept of *linkage* refers to the visibility of global symbols (such as variables, type names and function names) within the program as a whole across translation units. The concept of *scope* refers to symbols that are declared within a block such as a namespace, class, or function body. Such symbols are visible only within the scope in which they are defined; the concept of linkage does not apply to them.

## External vs. internal linkage

Non-const global variables and free functions by default have external linkage; they are visible from any translation unit in the program. You can override this behavior by explicity declaring them as **static** which limits their visiblity to the same translation unit in which they are declared. This meaning of **static** is different than its meaning when applied to local variables. Variables declared as **const** have internal linkage by default.

## Extern constexpr linkage

In earlier versions of Visual Studio, the compiler always gave a constexpr variable internal linkage even when the variable was marked extern. In Visual Studio 2017 version 15.5, a new compiler switch (/Zc:externConstexpr) enables correct standards-conforming behavior. Eventually this will become the default.

```cpp
extern constexpr int x = 10; //error LNK2005: "int const x" already defined
```

If a header file contains a variable declared extern constexpr, it needs to be marked **__declspec(selectany)** in order to correctly have its duplicate declarations combined:

```cpp
extern constexpr __declspec(selectany) int x = 10;
```

## See Also

 [Basic Concepts](../cpp/basic-concepts-cpp.md)