Pointer template with 'using' alias-template

Question | Nov 27, 2019 | nextptr 


The synonyms or typedef declarations are omnipresent in C++ codebases. They make the code readable and easier to maintain. Since C++11, we can create synonyms or aliases with either typedef or using keywords. These are some examples of defining aliases:

//aliases with typedef 
typedef std::string identifier_t;
typedef std::map<int,int> mymap_t;

//aliases with 'using'
using identifier_t = std::string;
using mymap_t = std::map<int,int>;

The synonyms created by the using keywords are called alias-declarations. One of the advantages of alias-declaration over the typedef is that the alias-declaration can be templatized, called alias-templates. Here is an example of alias-template:

template<typename T>
using VectorOfSets = std::vector<std::set<T>>;  

VectorOfSets<int> vs;

Pointer Template and The Question

One of the motivating examples of alias-templates is creating a pointer template, as shown below:

template<typename T>
using Ptr = T*;

We can use the above pointer template to declare a pointer:

struct Foo { /*...*/ };

void func(Ptr<int> pi) { /*...*/ } //A function parameter

Ptr<Foo> pf = new Foo(); //A Foo*

Ptr<int> pi = new int[100]; //An int*
func(pi); //Pass as argument

auto pc = (Ptr<char>)pi; //cast

Here is the question. A pointer 'p' is defined as int* const such that the pointer itself cannot point to another address once initialized:

int* const p = new int[100];

p = new int[100]; //ERROR

How can we define 'p' using the Ptr template? Select all the correct choices below (check Explanations for details):

Closing Words

The pointer template is simply one of the many compelling use cases of alias-templates. Another common application of alias-template is to hide template parameters:

//hide the key type
template<typename T>
using Map = std::map<int, T>;

//Create a std::map<int,std::string>
Map<std::string> mapStrings;  

The article 'How C++ using or alias-declaration is better than typedef' covers the benefits of alias-declaration over typedef.


Type alias, alias template - cppreference