将类的对象实例化数量限制为给定数量 [英] Restrict the number of object instantiations from a class to a given number

问题描述

给定一个类，我想将从该类创建的对象的数量限制为给定的数量，例如4.

Given a class, I would like to limit the number of objects created from this class to a given number, say 4.

有没有实现这一目标的方法?

Is there a method to achieve this?

推荐答案

基本思想是计算某个静态变量中已创建实例的数量.我会这样实现.虽然存在更简单的方法，但是这种方法有一些优点.

The basic idea is to count the number of created instances in some static variable. I would implement it like this. Simpler approaches exist, but this one has some advantages.

template<class T, int maxInstances>
class Counter {
protected:
    Counter() {
        if( ++noInstances() > maxInstances ) {
            throw logic_error( "Cannot create another instance" );
        }
    }

    int& noInstances() {
        static int noInstances = 0;
        return noInstances;
    }

    /* this can be uncommented to restrict the number of instances at given moment rather than creations
    ~Counter() {
        --noInstances();
    }
    */
};

class YourClass : Counter<YourClass, 4> {
}

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