C++11中新特性之:initializer_list详解

C++11提供的新类型,定义在<initializer_list>头文件中。

template< class T >
class initializer_list;

先说它的用处吧,然后再详细介绍一下。

首先有了initializer_list之后,对于STL的container的初始化就方便多了,比如以前初始化一个vector需要这样:

int a[] = {0, 1, 2, 3};
std::vector<int> vec(a, a+sizeof(a));

或者

std::vector<int> vec;
vec.push_back(1);
vec.push_back(3);
vec.push_back(3);
vec.push_back(2);

有了initializer_list后,就可以直接像初始化数组一样:

class Test {

private:
    static std::map<string, string> const nameToBirthday = {
        {"lisi", "18841011"},
        {"zhangsan", "18850123"},
        {"wangwu", "18870908"},
        {"zhaoliu", "18810316"},
    };
}

当然啦,里面的std::map必须提供参数为initializer_list的构造函数如:

map( std::initializer_list<value_type> init,
     const Compare& comp = Compare(),
     const Allocator& alloc = Allocator() );

其实for(initializer: list)中如果list是个形如:{a, b, c...},那么其实list自动被构造成了initializer_list对象。

 

 

 

下面稍微介绍一下initializer_list

一个initializer_list当出现在以下两种情况的被自动构造:

  1. 当初始化的时候使用的是大括号初始化,被自动构造。包括函数调用时和赋值
  2. 当涉及到for(initializer: list),list被自动构造成initializer_list对象

也就是说initializer_list对象只能用大括号{}初始化。

拷贝一个initializer_list对象并不会拷贝里面的元素。其实只是引用而已。而且里面的元素全部都是const的。

下面一个例子可以帮助我们更好的理解如何使用initializer_list:

 

#include <iostream>
#include <vector>
#include <initializer_list>

using namespace std;

template <class T>
struct S {
    vector<T> v;
    S(initializer_list<T> l) : v(l){
        cout << "constructed with a " << l.size() << "-elements lists" << endl;
    }
    void append(std::initializer_list<T> l) {
        v.insert(v.end(), l.begin(), l.end());
    }

    pair<const T*, size_t> c_arr() const{
        return {&v[0], v.size()};
    }

};


template <typename T>
void templated_fn(T arg) {
    for (auto a : arg)
        cout << a << " ";
    cout << endl;
}

int main() {
    S<int> s = {1, 2, 3, 4, 5}; //automatically construct a initializer_list 
                                // object and copy it
    s.append({6, 7 , 8});         //list-initialization in function call

    cout << "The vector size is now " << s.c_arr().second << " ints:" << endl;

    for (auto n : s.v)
        cout << ' ' << n;
    cout << endl;

    cout << "range-for over brace-init-list: " << endl;
    
    for (auto x : {-1, -2, 03})   //// the rule for auto makes this ranged for work
        cout << x << " ";
    cout << endl;

    auto al = {10, 11, 12};  //special rule for auto

    cout << "The list bound to auto has size() = " << al.size() << endl;


    //templated_fn({1, 2, 3});   //compiler error! "{1, 2, 3}" is not an expressionit has no type, and so T cannot be duduced.

    templated_fn<initializer_list<int> > ({7, 8, 9}); //ok
    templated_fn<vector<int> >({3, 5, 7});           //also ok

    return 0;
}

 

 

 

 

 

Reference:

http://en.cppreference.com/w/cpp/utility/initializer_list

http://www.cppblog.com/liyiwen/archive/2009/07/26/91248.html

 

posted on 2015-02-07 15:12  lysuns  阅读(39983)  评论(0编辑  收藏  举报

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