Skip to main content

Generic container idioms

Developing generic containers in C++ can become complex if you want to develope truly generic containers (as much as they can get). Relaxing the requirements on type T is the key behind developing truly generic containers. There a few C++ idioms to actually achieve the "lowest denominator" possible with requirements on type T.

It is easy to come up with a generic stack which requires following operations defiend on type T: a default constructor, a copy constructor, a non-throwing destructor and a copy assignment operator. But thats too much!

The requirements can be reduced to the folloing list: a copy constructor and a non-throwing destructor.

To achieve this, a generic container should be able to allocate uninitialized memory and invoke constructor(s) only once on each element while "initializing" them. This is possible using following two techniques:

1. operator new:
void * mem = operator new (sizeof (T) * NUMBER_OF_ELEMENTS);

2. construct helper using placement new:
template <class T1, class T2>
void construct (T1 *p, const T2 &value) {
new (p) T1(value);
}

operator new allocates uninitialized memory. It is a fancy way of calling malloc.
The construct helper template function invokes placement new and in turn invokes a copy constructor on the initialized memory. The pointer p is supposed to be one of the uninitialized memory chunks allocated using operator new.

Moreover, pointers in the range [end, end_of_allocated_range) should not point to objects of type T, but to uninitialized memory. (end can be considered an iterator pointing at an element one past the last initialized element of the container)

When an element is removed from the container, destructot should be invoked on them. A destroy helper function can be helpful here as shown.

template <class T>
void destroy (T *p) {
p->~T();
}

Similarly, to delete a range, another overloaded destroy function which takes two iterators could be useful. It essentially invokes first destroy helper on each element in the sequence.

Please see More C++ gems for elaborate articles on this topic. (authors Hurb Sutter and Matthew H. Austern)

Comments

Sumant said…
Also see more idioms related to developing generic containers in the open content wikibook: "More C++ Idioms"

Popular Content

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in

Want speed? Use constexpr meta-programming!

It's official: C++11 has two meta-programming languages embedded in it! One is based on templates and other one using constexpr . Templates have been extensively used for meta-programming in C++03. C++11 now gives you one more option of writing compile-time meta-programs using constexpr . The capabilities differ, however. The meta-programming language that uses templates was discovered accidently and since then countless techniques have been developed. It is a pure functional language which allows you to manipulate compile-time integral literals and types but not floating point literals. Most people find the syntax of template meta-programming quite abominable because meta-functions must be implemented as structures and nested typedefs. Compile-time performance is also a pain point for this language feature. The generalized constant expressions (constexpr for short) feature allows C++11 compiler to peek into the implementation of a function (even classes) and perform optimization