The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science

Programming Languages and Tools:

CS:3210:0001

Lecture/Lab #27

Programming with C++

Class templates, template specialization

Warm up: iterator invalidation

What's wrong with the following program?

auto find( std::vector<int> v, int x )
{
    auto end = v.end();
    auto iter = v.begin();
    for ( ; iter != end; ++iter )
        if ( *iter == x )
            break;

    return iter;
}

int main() {
    std::vector<int> v = { 11, 22, 33, 44, 55, 66, 77, 99 };
    assert( find( v, 42 ) == v.end() );
}

https://repl.it/@agurtovoy/lab27-warmup

Class templates

Class template defines a blueprint for a family of classes.

To use a specific class instance of a class template, we must supply class template arguments in the angle brackets following the class template name:

https://repl.it/@agurtovoy/lab27-class-templates

std::vector<int>

In contrast to function templates, except for a couple of specific contexts, we have to explicitly provide the class template parameters; the compiler will not deduce them for us.

The act of generating a specific instance of a class template by the compiler is called template instantiation.

A member function of a class template is instantiated only if the program uses that member function.

Template specialization

How can we change class and/or function template implementation for specific (sets of) template arguments?

Problem:

https://repl.it/@agurtovoy/lab27-template-specialization

When instantiated with bool, our class point takes 2 bytes. Can we make it more space efficient?

Example 1:

template< typename T > struct point {
    point() : x(), y() {}
    point( T x, T y ) : x( x ), y( y ) {}

    T x;
    T y;
};

int main() {
    point<bool> p;
    assert( sizeof( p ) == 1 ); // possible?
}

Template specialization (cont.)

https://repl.it/@agurtovoy/lab27-template-specialization

Function zero_fill sets the value of all elements of vector to 0. Can we make it more efficient for, say, vector<int>?

Example 2:

template< typename T >
void zero_fill( vector<T>& v )
{
    for ( auto& x : v )
        x = 0;
}