Question

I’m implementing a template function which sets the values of the first n elements of a sequence pointed to by a parameter iterator to values returned by calls to a generator function.

I have the following code:

template <typename IteratorOut, typename SizeType, typename Generator> IteratorOut createSequence(IteratorOut out, SizeType n, Generator gen) {
    std::vector<Generator> vec;
    for (SizeType i = 1; i <= n; i++) {
        vec.push_back(gen());
    }
    std::copy(vec.begin(), vec.end(), out);

    return out;
}

I’m testing the function with the following:

int generate () {
    return 5;
}
...
std::vector<int> vec_int;

createSequence(vec_int.begin(), 5, generate);
for (std::vector<int>::iterator iter = vec_int.begin(); iter != vec_int.end(); iter++) {
    std::cout << *iter << std::endl;
}

I get “error: invalid conversion from ‘int (*)()’ to ‘int’”. Can you see what’s wrong with my template function?

Answer 1

The type of the vector is wrong:

std::vector<Generator> vec;

Here, Generator is a function, and you want a vector of ints instead:

std::vector<SizeType> vec; // SizeType is an euphemism for int

But maybe you don’t need a vector at all!

for (SizeType i = 1; i <= n; i++) {
    *out = gen(); // is this what you want?
    ++out;
}