C++ template function to check if a vector of contains the value?

user_6935630 0
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I had planned to implement a kind of INDEX function for all types similar as in FORTRAN. Would this be a correct solution? ** EDIT after comments **

template <typename T>
bool contains(std::vector<T>& vec, T value){
 if (std::any_of(vec.begin(), vec.end(), [value](T j) { return value == j; }))return true;
 return false;
}

NB There is a pitfall with float type comparison for generic implementation!

templates c++
2 Answers

Yes, that is a valid implementation, however I'd write it differently

template <std::ranges::input_range R, typename T>
requires std::indirect_binary_predicate<ranges::equal_to, ranges::iterator_t<R>, const T*>
bool index(R&& range, const T & value){
    return std::ranges::find(range, value) != std::ranges::end(range);
}

This will work for any sequence and any value that can be compared for equality with elements of that sequence.

In C++11 there aren't concepts to be explicit about the requirements, but it goes similarly

template <typename R, typename T>
bool index(R&& range, const T& value) {
    using std::begin, std::end;
    return std::find(begin(range), end(range), value) != end(range);
}
user_26108100

  If the objective of your program is to find the determined value that is the correct way:

template <typename T>
bool index(std::vector<T>& vec, T value){
  return std::any_of(vec.begin(), vec.end(), [value](T j) { return value == j; });
}

  My program doesn't differ much from @caleth's answer as in order to write a general-purpose function that can handle various sorts of ranges, such as an std::array, std::vector, a C-like array, or others, you should write a template like this:

template <typename iter, typename T>
bool any_index(iter begin, iter end, T value) {
    for (; begin != end; ++begin) {
        if (static_cast<T>(*begin) == value) {
            return true;
        }
    }
    return false;
}

usage:

int main() {

    vector<int> v1{ 4, 3, 2, 1 };    
    vector<string> v2{ "hey", "language", "compiler", "bug" };
    vector<char> v3{ 'a', 'b','c', 'd' };
        
    array<int, 6> c{ {1,2,3,4,5,6} };
    unsigned char buf[5] = {0x11, 0x22, 0x33, 0x44, 0x55};
    
    assert(any_index(begin(v1), end(v1), 4) == true);
    assert(any_index(begin(v2), end(v2), string("compiler")) == true);
    assert(any_index(begin(v3), end(v3), 'd') == true);
    assert(any_index(begin(c), end(c), 5) == true);
    assert(any_index(begin(buf), end(buf), 0x11) == true);
          
    return 0;
}

  Full example (godbolt)

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