What does template <unsigned int N> mean? - c++

Question

When declaring a template, I am used to having this kind of code:
template <class T>
But in this question, they used:
template <unsigned int N>
I checked that it compiles. But what does it mean? Is it a non-type parameter? And if so, how can we have a template without any type parameter?

Answer 1

Yes, it is a non-type parameter. You can have several kinds of template parameters
Type Parameters.
Types
Templates (only classes and alias templates, no functions or variable templates)
Non-type Parameters
Pointers
References
Integral constant expressions
What you have there is of the last kind. It's a compile time constant (so-called constant expression) and is of type integer or enumeration. After looking it up in the standard, i had to move class templates up into the types section - even though templates are not types. But they are called type-parameters for the purpose of describing those kinds nonetheless. You can have pointers (and also member pointers) and references to objects/functions that have external linkage (those that can be linked to from other object files and whose address is unique in the entire program). Examples:
Template type parameter:
template<typename T>
struct Container {
T t;
};
// pass type "long" as argument.
Container<long> test;
Template integer parameter:
template<unsigned int S>
struct Vector {
unsigned char bytes[S];
};
// pass 3 as argument.
Vector<3> test;
Template pointer parameter (passing a pointer to a function)
template<void (*F)()>
struct FunctionWrapper {
static void call_it() { F(); }
};
// pass address of function do_it as argument.
void do_it() { }
FunctionWrapper<&do_it> test;
Template reference parameter (passing an integer)
template<int &A>
struct SillyExample {
static void do_it() { A = 10; }
};
// pass flag as argument
int flag;
SillyExample<flag> test;
Template template parameter.
template<template<typename T> class AllocatePolicy>
struct Pool {
void allocate(size_t n) {
int *p = AllocatePolicy<int>::allocate(n);
}
};
// pass the template "allocator" as argument.
template<typename T>
struct allocator { static T * allocate(size_t n) { return 0; } };
Pool<allocator> test;
A template without any parameters is not possible. But a template without any explicit argument is possible - it has default arguments:
template<unsigned int SIZE = 3>
struct Vector {
unsigned char buffer[SIZE];
};
Vector<> test;
Syntactically, template<> is reserved to mark an explicit template specialization, instead of a template without parameters:
template<>
struct Vector<3> {
// alternative definition for SIZE == 3
};

Answer 2

It's perfectly possible to template a class on an integer rather than a type. We can assign the templated value to a variable, or otherwise manipulate it in a way we might with any other integer literal:
unsigned int x = N;
In fact, we can create algorithms which evaluate at compile time (from Wikipedia):
template <int N>
struct Factorial
{
enum { value = N * Factorial<N - 1>::value };
};
template <>
struct Factorial<0>
{
enum { value = 1 };
};
// Factorial<4>::value == 24
// Factorial<0>::value == 1
void foo()
{
int x = Factorial<4>::value; // == 24
int y = Factorial<0>::value; // == 1
}

Answer 3

You templatize your class based on an 'unsigned int'.
Example:
template <unsigned int N>
class MyArray
{
public:
private:
double data[N]; // Use N as the size of the array
};
int main()
{
MyArray<2> a1;
MyArray<2> a2;
MyArray<4> b1;
a1 = a2; // OK The arrays are the same size.
a1 = b1; // FAIL because the size of the array is part of the
// template and thus the type, a1 and b1 are different types.
// Thus this is a COMPILE time failure.
}

Answer 4

A template class is like a macro, only a whole lot less evil.
Think of a template as a macro. The parameters to the template get substituted into a class (or function) definition, when you define a class (or function) using a template.
The difference is that the parameters have "types" and values passed are checked during compilation, like parameters to functions. The types valid are your regular C++ types, like int and char. When you instantiate a template class, you pass a value of the type you specified, and in a new copy of the template class definition this value gets substituted in wherever the parameter name was in the original definition. Just like a macro.
You can also use the "class" or "typename" types for parameters (they're really the same). With a parameter of one of these types, you may pass a type name instead of a value. Just like before, everywhere the parameter name was in the template class definition, as soon as you create a new instance, becomes whatever type you pass. This is the most common use for a template class; Everybody that knows anything about C++ templates knows how to do this.
Consider this template class example code:
#include <cstdio>
template <int I>
class foo
{
void print()
{
printf("%i", I);
}
};
int main()
{
foo<26> f;
f.print();
return 0;
}
It's functionally the same as this macro-using code:
#include <cstdio>
#define MAKE_A_FOO(I) class foo_##I \
{ \
void print() \
{ \
printf("%i", I); \
} \
};
MAKE_A_FOO(26)
int main()
{
foo_26 f;
f.print();
return 0;
}
Of course, the template version is a billion times safer and more flexible.