非模板类的显式类实例化

Question

In C++, you can explicitly instantiate a templated class like this:

template class A<B>;

This forces the compiler to include all of that class's methods its output, which is very useful for assessing which ones are covered by tests.

While (as I understand it) non-templated classes technically get instantiated automatically, unused methods of non-templated classes are often left out of the compiler's output (even with -O0 and other similar flags). Is there a way to force the compiler not to do that?

Answer 1

There is no concept of instantiation with non template classes. When the compiler encounters the definition of a member function, the definition is compiled right away.

Whether the function is outputted in the binary is up to the compiler and your setup. If you compile your program as a library, all functions will be implemented. If you use static linking, the linker won't add unused functions in you binary.

On the contrary, if you use dynamic linking, all exported definition will be there. Although non exported function such as function with static linkage can be inlined without having a separated definition.

Whether they are actually saved and outputted into the binary is something separated from the compilation itself. For example you could instantiate 100 template functions and in reality have none outputted in the binary. It's up to the linker if your function make it to the binary.

If your function is a public symbol, it will likely get into the binary.

Answer 2

I didn't manage to get a solution working with dynamic linking (the reason I write header-only libraries is that I hate linkers with a firey passion), but along the way I discovered some built-in compiler options in Clang that did what I want. I added the -fprofile-instr-generate -fcoverage-mapping flags to my compilation, and then used llvm-profdata and llvm-cov to view the test coverage (as described here ).

Note for anyone else trying to make this work: make sure to use the same llvm version for all three of these commands (clang, llvm-profdata, and llvm-cov).