auto is not allowed here in C++ VS2017

Question

bool isEven(int val) {
    return val % 2 == 0;
}

bool isOdd(int val) {
    return val % 2 != 0;
}

template<class Iterator>
int count_function(Iterator start, Iterator end, auto criteria) {
    int count = 0;
    for (; start != end; ++start) {
        if (criteria(*start)) {
            count++;
        }
    }
    return count;
}

Above is my code, auto before criteria is giving error "auto is now allowed here". I want to supply isEven /isOdd criteria to this function.

Why is that?

I have tried int, bool - that return some more problem.

Related: [auto parameter type in functions](https://stackoverflow.com/questions/31648458/auto-parameter-type-in-functions) — Remy Lebeau, Jul 07 '19 at 19:11

score 7 · Answer 1 · answered Jul 07 '19 at 06:16

7

The keyword auto is not allowed in function parameter. You need to use template if you want to use different datatypes.

template<class Iterator, class T>
int count_function(Iterator start, Iterator end, T criteria) {
    int count = 0;
    for (; start != end; ++start) {
        if (criteria(*start)) {
            count++;
        }
    }
    return count;
}

answered Jul 07 '19 at 06:16

SolidMercury

973
6
15

"The keyword auto is not allowed in function parameter." - It is in lambda functions. – Jesper Juhl Jul 07 '19 at 11:55

score 6 · Answer 2 · answered Jul 07 '19 at 06:21

Auto isn't allowed in normal function arguments. Its only allowed in lambda arguments. C++20 is going to add this functionality :)

Also look at "Abbreviated function template", here:

https://en.cppreference.com/w/cpp/language/function_template#Abbreviated_function_template

For now, you might get away with declaring your function with a lambda:

auto count_function = [](auto start, auto end, auto criteria)
{
    int count = 0;
    for (; start != end; ++start) {
        if (criteria(*start)) {
            count++;
        }
    }
    return count;
};

auto is not allowed here in C++ VS2017

2 Answers2