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?

@Nick That wont work since the parameter is only determined at runtime so I cannot list all possible functions in an array — mmirzadeh
– mmirzadeh, Commented Nov 5, 2012 at 7:42
but I cannot pass the functor to the c library! It only understands objects of type api_fun_t. I essentially could implement the nested function in a class. Problem is both parameter and the class method need to be static ... — mmirzadeh
– mmirzadeh, Commented Nov 5, 2012 at 7:51
I'm using c++ and am ok with templates! I'm just wondering if there are ways of doing this. — mmirzadeh
– mmirzadeh, Commented Nov 5, 2012 at 7:54

SomeWittyUsername · Accepted Answer · 2012-11-05 07:54:18Z

2

Use C++11 lambdas instead of nested functions. E.g.:

typedef int (*api_fun_t)(int);

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

edited Nov 5, 2012 at 7:54

answered Nov 5, 2012 at 7:37

SomeWittyUsername

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6 Comments

mmirzadeh Over a year ago

Can you show me an example of this? I'm interested to know how that would work

SomeWittyUsername Over a year ago

See here for more details: stackoverflow.com/questions/4726768/returning-functions

ymett Over a year ago

I don't see how that will help. The lambda can't access param.

SomeWittyUsername Over a year ago

@ymett no need: if(param == val1) return lambda1; if(param == val2) return lambda2;... Note, even if nested functions where allowed, accessing param from the nested function wouldn't have been possible for the same reasons as in lambda (requires state, which function pointer obviously doesn't have).

ymett Over a year ago

That's no better than an array of function pointers, which the questioner rejected in a comment to the question - "That wont work since the parameter is only determined at runtime so I cannot list all possible functions in an array".

|

ymett · Accepted Answer · 2012-11-05 09:55:18Z

0

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.

answered Nov 5, 2012 at 9:55

ymett

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