How to read a file from a specific directory within a CMake project and without hardcoded paths

Question

Intro

I have a subdirectory inside my src/-directory called game_engine. In game_engine I place game_engine-related classes which needs the content of files like shaders or OpenCL/Cuda-kernels that are placed in another or the same directories.

Problem

How to read the content of a file from a C++-class without using hardcoded absolute paths? The returned content must be a C-Style string.

What I have did so far?

I programmed the following helper function

char* read_source_code(const char *filename) {
    std::fstream file(filename);
    std::string content((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
    return const_cast<char *>(content.c_str());
}

This only seems to work if I pass the absolute filename. In addition, my IDE displays the following warning: Address of stack memory associated with local variable 'content' returned

I also tried to construct the absolute path at runtime, but the functions that I used returned me the path to the CMake-debug directory. So I decided to ask stackoverflow for help.

Additional information

• I want to call the function read_source_code inside any class that needs the content of another file placed in arbitrary directory of my CMake project.
• Hard coded file paths are not allowed, because it has to work also for other teammates.
• I don't want the warning to appear anymore because it seems legitimate. I probably programmed a bug.