Strange typenames and constructor in a template

Question

I am trying to understand a template class in C++. First, I would like to understand what this line means:

template <typename T, typename Ord = columns, typename All = abc::allocator<T,16> >
class matrix

where columns and allocator are respectively a struct and a class defined somewhere else (the second in the namespace abc). What troubles me is the fact that it seems to have a typename which has already been initialized. What does this mean? Should I also initialize the typename of Ord and All when I want to use this template?

Besides, there is also this only constructor:

explicit matrix(unsigned int rows = 0, unsigned int cols = 0, T init = T())

but it seems to have already been initialized. And what should init mean?

I assure you that I looked at all the code, but there is nothing that helps to understand better. Thank you for your attention.

Edit: Thank you everybody for your answers. Just a little reassurance (I am a noob in C++):

int const& operator() operator()(unsigned int i, unsigned int j) const

This method means that, when we initialize the class foo, we can call it by foo()(1,2), where i=1 and j=2. Am i right? And what do the two "const" refer to?

Thank you again!

score 1 · Answer 1 · answered Oct 21 '12 at 15:06

1

It means that the user of the class has set a reasonable default value- as in, you can provide one, but you don't have to. The same is of the constructor arguments. As for what the arguments mean, only you can answer that.

answered Oct 21 '12 at 15:06

Puppy

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Thank you for your answer. However, T() (which is defined only there) seems that init should be an empty "something". Am I correct? – Pippo Oct 21 '12 at 15:09
@Pippo The expression `T()` returns a value-initialized object of type `T` – zero for the numeric types, and a default-constructed object for class types. – Philipp Oct 21 '12 at 15:35
If it has a default constructor. Else, it will value initialize each member – Puppy Oct 21 '12 at 22:25

score 1 · Accepted Answer · edited Oct 21 '12 at 16:26

template <typename T, typename Ord = columns, typename All = abc::allocator<T,16> >
class matrix
{ 
    //...
};

Those are default template parameters, they work just as default function arguments - you can specify them, but if you don't, they are defaulted.

And you can see an example of usage of default arguments of the function.

Bottom line - all the following lines are correct:

matrix<int> a; // matrix<int, columns, abc::allocator<int, 16> >
matrix<int, rows> b; // matrix<int, rows, abc::allocator<int, 16> >
matrix<int, columns, abc::other_allocator<int, 32> > c; // obvious

matrix<int> a = matrix<int>(); // constructor called with 0, 0 and 
// int() - default constructed T - in this case, int -  as arguments
matrix<int> a(1, 2); // constructor called with 1, 2 and int() as arguments
matrix<int> a(1, 2, 100); // obvious

score 0 · Answer 3 · answered Oct 21 '12 at 15:08

0

It's a default value. If the the template argument isn't specified, it will take the default.
Just like you have default values in functions:

void blah(int a = 0) { }

answered Oct 21 '12 at 15:08

Yochai Timmer

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score 0 · Answer 4 · answered Oct 21 '12 at 15:10

As you know in C++ function arguments can have default values and in case that user do not provide that parameters, then C++ compiler use that default value for them. Now in constructor init has a default value of T() which means value of type using its default constructor, for example if T=int then T() mean 0 and if it is an std::string it is an empty string. You can even use this syntax for other parameters:

explicit matrix(
    unsigned int rows = unsigned int(),
    unsigned int cols = unsigned int(),
    T init = T());

Strange typenames and constructor in a template

4 Answers4