Clever Geek Handbook

C ++ code constructor syntax

I apologize in advance, my C ++ is rusty ...

What is he doing

: m_nSize(sizeof(t1))

mean in the next section?

class CTypeSize
{
   public:
      template<class T>
      CTypeSize(const T &t1) :
      m_nSize(sizeof(t1))
      {
      }
      ~CTypeSize(void){ };
      int getSize(void) const{ return m_nSize; }
   private:
      const int m_nSize;
};

I understand copy constructors, but I remember the syntax as Class :: Class (const Class & p). Am I thinking of something else or is this an alternative syntax?

Thank!

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5 answers


Nothing to do with the copy of the ctor. You initialize the m_nSize variable using an initialization list with the size of the template argument t1.



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It is not possible to initialize a member variable directly in the defination class. This is called the initialization list. You can imagine something like:

const int m_nSize = sizeof(t1);


C ++ 0x allows the above form.

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CTypeSize (const T & t1) is the constructor of the class. Class members can be initialized in the constructor.

class testclass {// constructor: a, b, c are set to // 0, 1, 2 testclass (): a (0), b (1), c (2) {}

int a, b, c; // members};

In your example: ": m_nSize (sizeof (t1))" means that m_nSize is initialized to sizeof (t1).

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Your question is double:

The syntax : member( value )

initializes a new member of the object value

.

But template< typename T> Class( const T& )

it is not a copy constructor. This is Class( const Class& )

.

So

#include <iostream>
struct C {
   template< typename T >
   C( const T& t ) { std::cout << "T"; }

  // C( const C& c ) { std::cout << "C"; }
};

int main() { C c1(1); C c2(c1); }

The template constructor will be called, followed by the "synthesized" copy constructor (only "T" will be output.)

When you insert a copy constructor explicitly, this call will be called (the output will be "TC").

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There is one more important thing about declaring a member variable CTypeSize::m_nSize

. Have you noticed a modifier const

in this declaration?

class member-var declared as "const"

can only be initialized in the initialization list.

As AraK mentioned, in C ++ 11 const member-var can also be initialized with a const expression. This is a compile-time case, while an initialization list allows const member-var to be initialized at runtime.

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