Passing variable into lambda

Question

I have a function that takes as an argument CallbackType, which is

typedef (*void) (const *char, bool)

I need to pass additional context, so I thought it would be a good idea to use lambdas:

CallbackType DelegateHandler(Pointer ptr) {
    return [&](const char* a, bool b) {
        callback(ptr, a, b); 
    };
}

It only works though if it's a capturing lambda, and a capturing lambda cannot be converted to a regular function pointer, so I get the error:

no known conversion for argument 1 from SetHandler(Pointer)::<lambda(const char*, bool)>’ to ‘CallbackType’ {aka ‘void(*)(const char*, bool)’

Any idea how to solve this in a simple fashion?

https://stackoverflow.com/questions/28746744/passing-capturing-lambda-as-function-pointer — Cory Kramer, May 07 '19 at 17:28
Can you share the code link here in comment with some online compiler? It would be helpful to debug — Mayur, May 07 '19 at 17:43

magras · Answer 1 · 2019-05-08T10:18:29.993

3

As Michael already pointed out, closures can not be converted to function pointers.

It might be better to accept templated argument or std::function instead of raw function pointer, if there is such an option.

edited May 08 '19 at 10:18

answered May 07 '19 at 17:31

magras

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@Mayur, without OP's code (part that accepts and calls callback) it would be a guess game. – magras May 08 '19 at 10:16

score 0 · Answer 2 · answered May 07 '19 at 17:26

Lambda functions that contain captures are not convertible to normal function pointers. Check this and this SO question for more.

Generally, you have a callback that allows you to include a custom pointer which is passed, which will contain the extra data you want from the callback.

Passing variable into lambda

2 Answers2