c++ - Why does a class used as a value in a STL map need a default constructor in ...?

Question

Below is the class used as the value in a map:

class Book
{
    int m_nId;
public:
    // Book() { }  <----- Why is this required?
    Book( int id ): m_nId( id ) { }

};

Inside main():

map< int, Book > mapBooks;

for( int i = 0; i < 10; ++i )
{
    Book b( i );
    mapBooks[ i ] = b;
}

The statement causing the error is:

mapBooks[ i ] = b;

If I add a default constructor, the error does not appear. However, I don't understand why the need. Can anyone explain? If I use insert(), the problem does not appear.

By the way, I'm using Visual C++ 2008 to compile.

Answer 1

operator[]执行两步过程。首先它为给定的键找到或创建一个映射条目，然后它返回对条目的值部分的引用，以便调用代码可以读取或写入它。

在之前不存在条目的情况下，条目的一半值需要在分配之前默认构造。这只是接口需要工作以与条目已经存在的情况保持一致的方式。

如果需要在地图中使用这种类型，那么您必须避免使用operator[]by usingfind和insert“手动”。