标准库里的count_if可以统计容器中满足特定条件的元素的个数。例如要统计一个整数vector——ivec中正数的个数,可以先写一个返回类型为bool,含有一个int参数的条件函数:
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之后可以用count_if(ivec.begin(),ivec.end(),pred)计算出正整数的个数。但这个方法有一个明显的缺陷:如果要统计大于10的个数、大于100的个数……就要写很多个类似的函数,能不能进一步抽象?如果能像这样调用count_if:count_if(ivec.begin(),ivec.end,pred(n)),就能统计出容器里大于n的个数,那该多好呀。函数对象提供了这样一种机制。
简单地说,函数对象就是一个重载了()运算符的对象,它可以像一个函数一样使用。例如这个Add类:
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结果将是4。那么这种机制怎样用在刚才的问题上呢?因为count_if的第三个参数是一个接受一个参数的函数,所以可以通过一个构造函数将这个参数传到对象里,由count_if自动调用这个函数对象:
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于是,要统计大于100的个数,就可以写成:count_if(ivec.begin(),ivec.end(),Pred(100))。Pred(100)构造一个函数对象,count_if把这个对象依次应用到容器中的每一个元素,只要它返回一个真值,计数器就加1。
如果再利用模板技术,则可以应用到各种类型的容器上:
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这样,刚才的统计语句就应该写成:count_if(ivec.begin(),ivec.end(),Pred<int>(100))。如果容器是double类型的,只需把尖括号里的int换成double即可。
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