C++ – Struct with template variables in C++

cclassstructtemplates

I'm playing around with templates. I'm not trying to reinvent the std::vector, I'm trying to get a grasp of templateting in C++.

Can I do the following?

template <typename T>
typedef struct{
  size_t x;
  T *ary;
}array;

What I'm trying to do is a basic templated version of:

typedef struct{
  size_t x;
  int *ary;
}iArray;

It looks like it's working if I use a class instead of struct, so is it not possible with typedef structs?

Best Answer

The problem is you can't template a typedef, also there is no need to typedef structs in C++.

The following will do what you need

template <typename T> 
struct array { 
  size_t x; 
  T *ary; 
};

Related Solutions

.net – What’s the difference between struct and class in .NET

In .NET, there are two categories of types, reference types and value types.

Structs are value types and classes are reference types.

The general difference is that a reference type lives on the heap, and a value type lives inline, that is, wherever it is your variable or field is defined.

A variable containing a value type contains the entire value type value. For a struct, that means that the variable contains the entire struct, with all its fields.

A variable containing a reference type contains a pointer, or a reference to somewhere else in memory where the actual value resides.

This has one benefit, to begin with:

value types always contains a value
reference types can contain a null-reference, meaning that they don't refer to anything at all at the moment

Internally, reference types are implemented as pointers, and knowing that, and knowing how variable assignment works, there are other behavioral patterns:

copying the contents of a value type variable into another variable, copies the entire contents into the new variable, making the two distinct. In other words, after the copy, changes to one won't affect the other
copying the contents of a reference type variable into another variable, copies the reference, which means you now have two references to the same somewhere else storage of the actual data. In other words, after the copy, changing the data in one reference will appear to affect the other as well, but only because you're really just looking at the same data both places

When you declare variables or fields, here's how the two types differ:

variable: value type lives on the stack, reference type lives on the stack as a pointer to somewhere in heap memory where the actual memory lives (though note Eric Lipperts article series: The Stack Is An Implementation Detail.)
class/struct-field: value type lives completely inside the type, reference type lives inside the type as a pointer to somewhere in heap memory where the actual memory lives.

C++ – When should you use a class vs a struct in C++

The differences between a class and a struct in C++ is:

struct members and base classes/structs are public by default.
class members and base classes/struts are private by default.

Both classes and structs can have a mixture of public, protected and private members, can use inheritance and can have member functions.

I would recommend you:

use struct for plain-old-data structures without any class-like features;
use class when you make use of features such as private or protected members, non-default constructors and operators, etc.

Best Answer

Related Solutions

.net – What’s the difference between struct and class in .NET

C++ – When should you use a class vs a struct in C++

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