Question

I am getting compilation error is following code. I thought this should have worked in c++
Can anybody help me to understand what is wrong here.

template < typename elem_type>
elem_type *find2( std::vector<elem_type>& vec, elem_type value) {
    for ( int i = 0; i < vec.size(); ++i) {
        if ( vec[i] == value ) {
            return &vec[i];
        }
    }
    return 0;
}
int main( int argc, char **argv) {
    int arr[10] = {1,2,3,4,5,6,7,8,9,10};
    std::vector<int> vec( arr, arr+10);     
    int value = 9;
    int *ptr1 = find2(vec,value);
}

following is the compilation error

1>          d:\personal\work\find\find\find.cpp(25) : see reference to function template instantiation 'elem_type *find2<int>(std::vector<_Ty> &,elem_type &)' being compiled
1>          with
1>          [
1>              elem_type=int,
1>              _Ty=int
1>          ]

compiler is Visual Studio 11

Answer 1

In this line,

    for ( int i = 0; i < vec.size(); ++i ) {

you are comparing the signed int variable i to the unsigned size_t result of vec.size().

The compiler warns because such a comparision is unsafe in the face of implicit promotions in C++. i is promoted to size_t. If i were (hypothetically) negative, that would produce a really huge value, and would thus produce an unexpected comparision result.

A simple cure is to

    #include <stddef.h>
    typedef ptrdiff_t Size;
    typedef Size Index;

and then do e.g.

    for ( int i = 0; i < Size( vec.size() ); ++i ) {

You will probably get at least one answer recommending the apparently simpler

    for ( size_t i = 0; i < vec.size(); ++i ) {

but this is problematic for the same reason that the compiler warned: using unsigned integers as numbers risks getting very weird and unexpected results, buggy results, due to implicit promotions and in general conversion from negative number to unsigned, or vice versa.

Even better than the casting above, define a countOf function like

    template< class Container >
    Size countOf( Container const& c ) { return v.size(); }

    template< class Elem, Size n >
    Size countOf( Elem (&)[n] ) { return n; }

and then write just

    for ( int i = 0; i < countOf( vec ); ++i ) {

And best, forget about that indexing and use iterators:

    for ( auto it = vec.begin(); it != vec.end(); ++it ) {