Making class constructor private

Question

I am writing a simple garbage collector in C++. I need a singleton class GarbageCollector to deal with different types of memory. I used a Meyer's singleton pattern. But when I try to call instance, an error appears:

 error: ‘GarbageCollector::GarbageCollector(const GarbageCollector&)’ is private
    GarbageCollector(const GarbageCollector&);
    ^

Here is the class definition.

class GarbageCollector //Meyers singleton (http://cpp-reference.ru/patterns/creational-patterns/singleton/)
{
 public:
    static GarbageCollector& instance(){
        static GarbageCollector gc; 
        return gc; 
    }   
    size_t allocated_heap_memory;
    size_t max_heap_memory;
private:
    //Copying, = and new are not available to be used by user.
    GarbageCollector(){};
    GarbageCollector(const GarbageCollector&);
    GarbageCollector& operator=(GarbageCollector&);
};

I call the instance with the following line: auto gc = GarbageCollector::instance();

In your `class` there is a comment: `Copying,[...] are not available [...]`. You are getting the error because you are copying the gc — Rakete1111, May 15 '16 at 15:37
Everything is said in the error message `GarbageCollector(const GarbageCollector&);` is private. You cannot call a private constructor from outside the class. — Ely, May 15 '16 at 15:37
@Elyasin: Not everything. The surprising bit is that the `auto` variable is declared as a `GarbageCollector`, not `GarbageCollector&`. — Martin Bonner supports Monica, May 15 '16 at 15:56
Given you are using `auto`, you might as well delete the copy constructor and assignment, rather than just declaring them private - then you can't copy even *inside* the class. — Martin Bonner supports Monica, May 15 '16 at 15:57

songyuanyao · Accepted Answer · 2016-05-15T15:45:29.317

3

Change

auto gc = GarbageCollector::instance();

to

auto& gc = GarbageCollector::instance();

Otherwise gc is not a reference, then returned GarbageCollector need to be copied, but the copy ctor is private, that's why compiler complains.

edited May 15 '16 at 15:45

answered May 15 '16 at 15:38

songyuanyao

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Making class constructor private

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