Access calling function's local variable

Question

This is a toy example of the issue. I have a large parent function, which, among other things, calls two child functions. In reality, this parent function is on a base class used for other things, so I don't want to rewrite the parent function or the wrapped functions to pass variables by reference, because they won't be needed for the other child classes inheriting the base. The parentFunc is also being called on multiple threads, so I can't just stick the needThisInSecondWrappedFunc variable as a class-level variable, as it would then be incorrectly shared between threads.

It seemed to me making a local variable on the parent function would be visible to the two child functions, which could then operate on the parentFunc's local variable, but it's not the case.

#include <iostream>

void parentFunc(float& data);
void wrappedFunc(float& ref);
void secondWrappedFunc(float& ref);

void parentFunc(float& data)
{
float needThisInSecondWrappedFunc[3];
wrappedFunc(data);
secondWrappedFunc(data);
}

void wrappedFunc(float& ref)
{
    needThisInSecondWrappedFunc[0] = ref * 0.5f;
    needThisInSecondWrappedFunc[1] = ref * 0.5f;
    needThisInSecondWrappedFunc[2] = ref * 0.5f;
}

void secondWrappedFunc(float& ref)
{
    ref = needThisInSecondWrappedFunc[0] +
          needThisInSecondWrappedFunc[1] +
          needThisInSecondWrappedFunc[3];
}

int main()
{
    float g;
    g = 1.0f;
    parentFunc(g);
    std::cout << g << '\n';
    return 0;
}

I'm not sure why wrappedFunc and secondWrappedFunc can't see the local variables of the parentFunc - I thought the parentFunc locals would still be in scope at this point?

Matthieu Brucher · Accepted Answer · 2018-12-29 18:50:05Z

2

There is no concept of parent function access in C++.

You can only access the global scope ("global" variables) and then local variables inside the current function. If you are in an object instance, you also have access to these.

There is no way to access variables declared in another function.

What you need to do is this:

void parentFunc(float& data);
void wrappedFunc(float& ref, float* needThisInSecondWrappedFunc);
void secondWrappedFunc(float& ref, const float* needThisInSecondWrappedFunc);

void parentFunc(float& data)
{
float needThisInSecondWrappedFunc[3];
wrappedFunc(data, needThisInSecondWrappedFunc);
secondWrappedFunc(data, needThisInSecondWrappedFunc);
}

void wrappedFunc(float& ref, float* needThisInSecondWrappedFunc)
{
    needThisInSecondWrappedFunc[0] = ref * 0.5f;
    needThisInSecondWrappedFunc[1] = ref * 0.5f;
    needThisInSecondWrappedFunc[2] = ref * 0.5f;
}

void secondWrappedFunc(float& ref, const float* needThisInSecondWrappedFunc)
{
    ref = needThisInSecondWrappedFunc[0] +
          needThisInSecondWrappedFunc[1] +
          needThisInSecondWrappedFunc[3];
}

Or even better, use std::array<float, 3>.

answered Dec 29, 2018 at 18:50

Matthieu Brucher

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3 Comments

Charles Angus Over a year ago

Is there really no way around re-writing all three functions to explicitly pass in the param? As I said, this would mean modifiying a base class used in many other places which don't need this param just to special-case this one derived class...

Matthieu Brucher Over a year ago

Then you have the wrong design and needThisInSecondWrappedFunc should actually be a member variable of the derived class.

Charles Angus Over a year ago

Fair enough. Thanks for the quick help!

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