C++ – Disable copy constructor

ccopy-constructor

I have a class :

class SymbolIndexer {
protected:
  SymbolIndexer ( ) { }

public:
  static inline SymbolIndexer & GetUniqueInstance ( ) 
  { 
    static SymbolIndexer uniqueinstance_ ;
    return uniqueinstance_ ; 
  }
};

How should I modify it to disable code like:

SymbolIndexer symbol_indexer_ = SymbolIndexer::GetUniqueInstance ( );

and only allow code like :

SymbolIndexer & ref_symbol_indexer_ = SymbolIndexer::GetUniqueInstance ( );

Best Answer

You can make the copy constructor private and provide no implementation:

private:
    SymbolIndexer(const SymbolIndexer&);

Or in C++11, explicitly forbid it:

SymbolIndexer(const SymbolIndexer&) = delete;

Related Solutions

C++ – Does the ‘mutable’ keyword have any purpose other than allowing the variable to be modified by a const function

It allows the differentiation of bitwise const and logical const. Logical const is when an object doesn't change in a way that is visible through the public interface, like your locking example. Another example would be a class that computes a value the first time it is requested, and caches the result.

Since c++11 mutable can be used on a lambda to denote that things captured by value are modifiable (they aren't by default):

int x = 0;
auto f1 = [=]() mutable {x = 42;};  // OK
auto f2 = [=]()         {x = 42;};  // Error: a by-value capture cannot be modified in a non-mutable lambda

C++ – Storing C++ template function definitions in a .CPP file

The problem you describe can be solved by defining the template in the header, or via the approach you describe above.

I recommend reading the following points from the C++ FAQ Lite:

They go into a lot of detail about these (and other) template issues.

Best Answer

Related Solutions

C++ – Does the ‘mutable’ keyword have any purpose other than allowing the variable to be modified by a const function

C++ – Storing C++ template function definitions in a .CPP file

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