Make a Single Capturing Lambda

Question

This is obviously a toy example, but lets say I have a n functions like this:

void one(const int param) {
    const auto func = [=](){ return 13 == param; };
}

void two(const int param) {
    const auto func = [=](){ return 13 == param; };
}

And so on; they all have an identical capturing lambda. Is it possible to have 1 instance of the lambda which always captures the param of the function that it is in, rather than n instances? Maybe as a follow up question I should be asking, will the compiler already recognize the replication and simplify these to a single instance?

Answer 1

Unfortunately you will get multiple types with this solution. [expr.prim.lambda.closure]/1 states that

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.

^{emphasis mine}

So each

const auto func = [=](){ return 13 == param; };

is its own expression so you get a new unique type, even though they are syntactically the same.

What you could do is is factor the repetition out into a functor and then you would only have one class that is defined.

class compare
{
    int val;
public:
    compare(int val) : val(val) {}
    bool operator() const { return val = 13; }
};

and then your functions would become

void one(const int param) {
    const auto func = compare{param};
}

void two(const int param) {
    const auto func = compare{param};
}