When is a header file required for using a static library?

Question

If I create a static library in C++ in Linux such that a ".a" file is produced, how do I (or anyone else) use the library? For example, my library defines a class. I assume that it is not sufficient merely to provide the ".a" file, but also to provide a header file. How do I know what header files must be provided with the ".a" file?

For example, do I need to provide all header files that were included anywhere in the code for my library?

Answer 1

Header files provide the "declarations" for the classes and functions. These are needed by the compiler, so it can a) validate that you are passing the right parameters, and/or setting the right data members of classes / structs, and b) so it can know how to call those functions.

void do_something(int a, std::string& s);

tells the compiler that this function is expecting two parameters: an int, and a string&. This validates that you are passing the right types of parameters (a language-level construct), and it explains what the object code in the compiled library is expecting (two arguments -- how is determined by calling convention.)

If that compiled library is using code from another library, you do not have to provide those headers, because they have nothing to do with code that you've written. The libraries are working at a "application binary interface" (ABI) level, not an "application programming interface" (API). That means they're just passing around pointers, etc. Not parameters with C types.