Friday, January 17, 2014

non type class template parameters c++

non type class template parameters c++ : templates tutorial
In contrast to the sample implementations of a stack in previous chapters, you can also implement a stack by using a fixed-size array for the elements. An advantage of this method is that the memory management overhead, whether performed by you or by a standard container, is avoided. However, determining the best size for such a stack can be challenging. The smaller the size you specify, the more likely it is that the stack will get full. The larger the size you specify, the more likely it is that memory will be reserved unnecessarily. A good solution is to let the user of the stack specify the size of the array as the maximum size needed for stack elements.
To do this, define the size as a template parameter:

The new second template parameter, MAXSIZE, is of type int. It specifies the size of the array of stack elements:

In addition, it is used in push() to check whether the stack is full:

To use this class template you have to specify both the element type and the maximum size:

Note that each template instantiation is its own type. Thus, int20Stack and int40Stack are two different types, and no implicit or explicit type conversion between them is defined. Thus, one cannot be used instead of the other, and you cannot assign one to the other.
Again, default values for the template parameters can be specified:

However, from a perspective of good design, this may not be appropriate in this example. Default values should be intuitively correct. But neither type int nor a maximum size of 100 seems intuitive for a general stack type. Thus, it is better when the programmer has to specify both values explicitly so that these two attributes are always documented during a declaration.

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