Pass specialized function pointers in c++

Question

I am working a with a c library inside my c++ code. Their API requires me passing a certain function pointer with a given signature. Let's say it's something like the following:

typedef int (*api_fun_t)(int);

What I want to be able to do is to pass function pointers that depend on certain parameters that are determined at runtime. Initially I thought of defining something like the following:

api_fun_t my_specialized_fun(int param){
    int fun(int x){
        // decide what to do based on param
    }
    return fun;
}

but problem c++ does not allow nested function definition. Next I figured I can achieve this via templates like the following

template <int param>
fun (int x){
    //decide what to do based on param
}

Is there any other way of doing this that does not involve global and/or static variables?

Answer 1

Use C++11 lambdas instead of nested functions. Eg:

typedef int (*api_fun_t)(int);

api_fun_t my_specialized_fun(int param){
   return [](int x) {x++; return x;};
}

Answer 2

Assuming the API allows you to pass a user parameter (as all well designed C-style callback APIs do), use a struct to hold the data, and pass a pointer to it as the parameter. You can then use the member data of the struct or call a member function as appropriate. You will have to make sure the lifetime of the struct is long enough.

If the API does not allow you to pass a user parameter, your only option is a global variable, thread local if multithreading is an issue. And write an angry email to the designer of the API.