C++ unary function for logical true

Question

I'm trying to use the any_of function on a vector of bool's. The any_of function requires a unary predicate function that returns a bool. However, I can't figure out what to use when the value input into the function is already the bool that I want. I would guess some function name like "logical_true" or "istrue" or "if" but none of these seem to work. I pasted some code below to show what I am trying to do. Thanks in advance for any ideas. --Chris

// Example use of any_of function.

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>

using namespace std;

int main(int argc, char *argv[]) {
    vector<bool>testVec(2);

    testVec[0] = true;
    testVec[1] = false;

    bool anyValid;

    anyValid = std::find(testVec.begin(), testVec.end(), true) != testVec.end(); // Without C++0x
    // anyValid = !std::all_of(testVec.begin(), testVec.end(), std::logical_not<bool>()); // Workaround uses logical_not
    // anyValid = std::any_of(testVec.begin(), testVec.end(), std::logical_true<bool>()); // No such thing as logical_true

    cout << "anyValid = " << anyValid <<endl;

    return 0;
}

Answer 1

Looks like you want something like an identity function (a function that returns whatever value it is passed). This question seems to suggest no such thing exists in std:::

Default function that just returns the passed value?

In this case the easiest thing might be to write

bool id_bool(bool b) { return b; }

and just use that.

Answer 2

You can use a lambda (since C++11):

bool anyValid = std::any_of(
    testVec.begin(), 
    testVec.end(), 
    [](bool x) { return x; }
);

And here's a live example.

You can, of course, use a functor as well:

struct logical_true {
    bool operator()(bool x) { return x; }
};

// ...

bool anyValid = std::any_of(testVec.begin(), testVec.end(), logical_true());

And here's a live example for that version.

Answer 3

I ended up here looking for a C++ standard library symbol to do this:

template<typename T>
struct true_ {
    bool operator()(const T&) const { return true; }
};

which I think is what the op wants, and which can be used, e.g., as follows:

std::any_of(c.begin(), c.end(), std::true_);

I could not find anything like this in the standard library, but the struct above works and is simple enough.

While the any_of expression above makes no sense in isolation (it would always return true, unless c is empty), a valid use case of true_ is as a default template argument for a template class expecting a predicate.

Answer 4

As of C++20, we get std::identity, which could be helpful.