template<class IIter, class Predicate>
constexpr
IIter find_if(IIter first, IIter last, Predicate pred);

template <class ExecutionPolicy, class ForwardIterator, class Predicate>
ForwardIterator find_if(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, Predicate pred);

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
int main()
{
    std::string str = "hello world!";
    auto find_it = std::find_if(std::begin(str), std::end(str), [](auto s){ return ::isspace(static_cast<unsigned char>(s)); });
    if (find_it != std::end(str))
    {
        std::cout << "s:" << std::string_view(find_it, std::end(str)) << std::endl;
    }
    else
    {
        std::cout << "Нет пробелов!" << std::endl;
    }
    return 0;
}

s: world!