How to call function member with different argument several times?

Question

My class has function that follows simple pattern, yet I don't know if there is a way to avoid code duplication so it will be much shorter. Here is simplified example:

class Data
{
public:
    void compute()
    {
        if (procedure1)
        {
            runAlpha(param1);
            runAlpha(param2);
            runAlpha(param3);
        }
        if (procedure2)
        {
            runBeta(param1);
            runBeta(param2);
            runBeta(param3);
        }
        if (procedure3)
        {
            runGamma(param1);
            runGamma(param2);
            runGamma(param3);
        }
    }
}

runAlpha, runBeta, runGamma are also public members of this class. As you see, there is very little difference between each procedure and also between run calls. I would like to simplify this code a little bit, but im not allowed to modify run functions, so I thought about using templates. However, adding the callMultipleRuns method to class and replacing body of if statement doesn't compile.

template <typename Run>
void callMultipleRuns(Run r)
{
    r(param1);
    r(param2);
    r(param3);
}

void compute()
{
    if (procedure1)
        callMultipleRun(runAlpha);
    if (procedure2)
        callMultipleRun(runBeta);
    if (procedure3)
        callMultipleRun(runGamma);
}

The error is error: invalid use of non-static member function. I cannot add static to run declaration. Is there a way to fix it?

can param1, param2, etc. be in an std::array or std::vector? If so, this could be simplified by being able to iterate them, or to use other constructions that execute functions on ranges of values. — ttemple
– ttemple, Commented Oct 23, 2020 at 10:50
There is definitely a simple "way to fix it", but you need to show real code, a minimal reproducible example, instead of made-up code. The only possible answer to a question with fake code would be a fake answer, too. It would be a shame if someone were to spend their time to put together an answer, based on made-up code, only to discover that some minute detail was lost in translation, and the given answer won't work for some reason. I was going to write up one simple solution, but realized that I'll likely miss some detail that you haven't mentioned, which would make it moot. — Sam Varshavchik
– Sam Varshavchik, Commented Oct 23, 2020 at 11:05

JHBonarius · Accepted Answer · 2020-10-23 11:21:36Z

1

Firstly, you found that you cannot simply pass a non-static member function as a parameter. However you can pass a reference to the member function. This must be in a specific form, as specified by the standard.

Some example code

#include <cstdio>
class Data {
public:
    void runAlpha(int i) { printf("a%d", i); };
    void runBeta(int i) { printf("b%d", i); };
    void runGamma(int i) { printf("c%d", i); };

    template <typename Run>
    void callMultipleRuns(Run r)
    {
        (this->*r)(1);
        (this->*r)(2);
        (this->*r)(3);
    }

    void compute()
    {
        if (true) callMultipleRuns(&Data::runAlpha);
        if (true) callMultipleRuns(&Data::runBeta);
        if (true) callMultipleRuns(&Data::runGamma);
    }
};

int main() {
    Data{}.compute();
}

on Godbolt

edited Oct 23, 2020 at 11:21

answered Oct 23, 2020 at 11:12

JHBonarius

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Botje · Accepted Answer · 2020-10-23 11:12:35Z

0

You're close, but you cannot use runAlpha and friends as standalone functions like that; they need an instance.

Luckily, the C++ standard library has helpers for this.
The first is simply to pass a lambda function:

callMultipleRuns([=](auto param) {runAlpha(param);})

or use std::bind:

callMultipleRuns(std::bind(&Data::runAlpha, this, std::placeholders::_1);

answered Oct 23, 2020 at 11:12

Botje

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