2.2.2. Variable Declarations and Definitions
To allow programs to be written in logical parts, C++ supports what is commonly known as separate compilation. Separate compilation lets us split our programs into several files, each of which can be compiled independently.
When we separate a program into multiple files, we need a way to share code across those files. For example, code defined in one file may need to use a variable defined in another file. As a concrete example, consider std::cout and std::cin. These are objects defined somewhere in the standard library, yet our programs can use these objects.
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