将constexpr与getenv一起使用（或替代方法）

Question

I'm trying to capture environment variables from a build server as compile time constants that can be used within the build library. I have a static class that can be queried for these constants and would like to use constexpr with std::getenv , but I get the errors that constexpr variable must be initialised from a constant expression due to the fact that getenv returns a non-const char* , due to the fact it is a little long in the tooth .

I'd like to avoid bloating the build scripts by injecting all of the env vars as -DMY_ENV_VAR if possible. If the answer is simply "no, you must add each as a definition like that", and there are no modern alternatives to getenv or tricks I can use then so be it, but then there are 2 spots to maintain, which is not ideal.

Answer 1

The runtime environment of a C++ program, which getenv interacts with, is fundamentally not compile-time constant.

The char* vs const char* issue you pointed out has nothing to do with this problem.

Compile-time constants are things that are fixed at compile-time. Your runtime environment is not fixed at compile-time. So getenv 's return values cannot be constexpr .

You could create script that writes a header containing the build-time environment variables you want to store in constexpr storage, instead of a pile of -D commands.

Answer 2

I don't think std::getenv can be used as a constexpr since it's a runtime system call. What do you mean by "there would be 2 spots to maintain"? Your static class would just use the defines passed into build, ie

class CBuildConsts
{
    public:
    static const std::string Thing;
}

and then in the .cpp file

const std::string CBuildConsts::Thing = std::string("MY_ENV_VAR");