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Lec 8: Template Classes and Const Correctness

Motivation

You were working at a graphics startup in 1998, and you needed a simple data structure to manage Point objects.

With your knowledge of pointer arithmetic and classes you create this PointVector:

class PointVector {
public:
    // methods for adding,removing,accessing Points
    // take a look at the lecture code if intersted!
private:
    Point* elements;
    int size;
}

You realized that you not only need to keep track Point, but also need to keep track of bool, char, int, short, long, long long, unsigned int, unsigned char, unsigned short, unsigned long, unsigned long long, float, double, long double, std::size_t, ptrdiff_t, int8_t, int16_t, int32_t, int64_t, uint8_t, uint16_t, uint32_t, uint64_t, char16_t, char32_t, wchar_t, void, nullptr, and finally, enum types. That's A LOT of types!

Template Class

Definition: Template Class is a class that is parametrized over some number of types; it is comprised of member variables of a general type/types.

Example (IntContainer vs Container):

/*
* container.h
*/

/*
// This is not a container for all types,
// so we won't use this.
class IntContainer { 
public:
    IntContainer (int val);
    int getValue();

private:
    int value;
}
*/
#pragma once

// When making template classes you need to `#include` the `.cpp` implementation in the .h file. This is a compiler quirk
// — not super important for this class, but just keep it in mind when working with templates. 
#include "container.cpp"

template <typename T>
// This is a template
// declaration and allows us
// to create template classes
class Container {
public:
    IntContainer (T val);
    T getValue();

private:
    T value;
}

/*
* container.cpp
*/

#include <container.h>

template <typename T>
Container<T>::Container(T val) {
    this->value = val;
}

template <typename T>
T Container<T>::getValue () {
    return this->value;
}

Note:

  1. It's crucial to highlight that when we write member functions, we use the notation Container<T> instead of just Container. This is because Container<T> represents different classes for different types of T. For example, Container<int> is not the same as Container<float>. So, it's important not to mix them up.
  2. To correctly define a template with its associated parameters, you need to put the declaration template <typename T, ...> at the beginning of the function. This tells the compiler that we're using a template and specifies the types of the parameters that will be used.
  3. When used in templates, typename and class are interchangeable.
  4. When making template classes you need to #include the .cpp implementation in the .h file. This is a compiler quirk — not super important for this class, but just keep it in mind when working with templates.

Const Correctness

std::string stringify(const Student& s){
    return s.getName() + " is " + std::to_string(s.getAge()) +
        " years old." ;
}
// Compiler error!

In the absence of the const specifier for member methods, the compiler faces the challenge of determining the intrinsic const-ness of the methodsgetName and getAge.

So, we add const at the end of a member method that does not modify this.

  • Indeed, member methods possess a default argument - this - which, by default, is not considered const.
  • So, the "const correctness" is actually the const for this!
  • And, for sure, const objects can only interact with const-interface!

const_cast

const_cast performs operations on the low-levelconst (referred to as "casting away the const") in C++.

#include <iostream>
#include <string>

using namespace std;

const string& smaller(const string& a, const string& b) {
    // Returns the smaller of the two input strings, 'a' and 'b'.
    return (a < b ? a : b);
}

string& smaller(string& a, string& b) {
    // Returns the smaller of the two input strings, 'a' and 'b'.
    // We use `const_cast<const string&>` here to avoid a self loop.
    // Without `const_cast<const string&>`, the 'smaller' function will merely call itself,
    // i.e., the non-const version, instead of the const version.
    return const_cast<string&>(smaller(const_cast<const string&>(a), const_cast<const string&>(b)));
}

int main() {
    string a = "hello", b = "world";
    auto& c = smaller(a, b);
    cout << c + "!";
    return 0;
}

As mentioned above, we can achieve overloading for smaller function with the following implementations:

  • Return a const reference for a const argument.
  • Return a non-const reference for a non-const argument.
  • Only implement the main function once, and for the non-const version, simply call the const version - following the DRY principle!