How to convert void to typedef void(...)(float)

Question

I have this on one class:

typedef void(*PERCENTAGE_CALLBACK)(float);

And I use it on functions like this one:

int API_GenerateLayerData(int layerIndex, QByteArray data, int dataSize, PERCENTAGE_CALLBACK callBack);

But the thing is that I can't pass a parameter with a void return type and accepts float as a parameter like this:

void updateFormattingProcess(float value)
{
    emit ChangeCompose(int(value));
}

void someFunction()
{ 
    //It says cannot convert from 'void' to 'PERCENTAGE_CALLBACK'
    API_GenerateLayerData(1, data, count, updateFormattingProcess(x));
}

This is `updateFormattingProcess(x)` function call, all you need is to pass pointer of this function: `&updateFormattingProcess`. — rafix07, Jul 11 '19 at 07:43
how do I set the value of the float in the updateFormattingProcess ? — TerribleDog, Jul 11 '19 at 07:45
Is there a reason that you're not using [std::function](https://en.cppreference.com/w/cpp/utility/functional/function)? — Neijwiert, Jul 11 '19 at 07:47
refer to : [How do you pass a function as a parameter in C?](https://stackoverflow.com/questions/9410/how-do-you-pass-a-function-as-a-parameter-in-c) — Sander De Dycker, Jul 11 '19 at 07:53

score 3 · Accepted Answer · answered Jul 11 '19 at 07:43

3

You are trying to call the function with updateFormattingProcess(x) and pass the result of that function call (which is void) as a parameter of type void(*)(float). Instead, just pass a pointer to it:

API_GenerateLayerData(1, data, count, &updateFormattingProcess);
//                                    ^ take the addresss of the function

answered Jul 11 '19 at 07:43

lubgr

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how do I set the value of the float in the updateFormattingProcess ? – TerribleDog Jul 11 '19 at 07:46
Well, you don't. `API_GenerateLayerData` accepts a function pointer as a parameter. This suggests that it calls `updateFormattingProcess` internally and selects the right parameter?! – lubgr Jul 11 '19 at 07:46
it has another error '&' illegal operation bound member function expression – TerribleDog Jul 11 '19 at 07:48
So you call `API_GenerateLayerData` from inside a member function, and `updateFormattingsProcess` is a member function, too? – lubgr Jul 11 '19 at 07:49
3

@TerribleDog - Means your example is not a [mcve]. Please edit your question to **faithfully** represent your issue. – StoryTeller - Unslander Monica Jul 11 '19 at 07:49
2

This is probably because `updateFormattingProcess()` is non-static member of some class. You either need to do it static member or not member of any class. – sklott Jul 11 '19 at 07:51

How to convert void to typedef void(...)(float)

1 Answers1