c ++：实例化对象 [英] c++: instantiate object

问题描述

可能重复：

C ++对象实例化与赋值

C ++并且想知道在实例化一个对象之间有什么区别（如果有）

  int main（）{
载体， int> x（2）; 
} 
  

或

  int main（）{
 vector< int> x = int> （2）。 
} 
  

，除了后者需要更长时间写入。提前感谢！

解决方案

差异主要是语法：

• 矢量< int>




• 向量< int> ; x = vector< int>（2）; 是复制初始化。

后者正式要求类具有可访问的复制构造函数，但实际上，复制将被省略，并且两个版本产生完全相同的代码。

• 
  
  < > vector< int> x = vector< int>（vector< int>（vector< int>（2）））;

Possible Duplicate:
C++ Object Instantiation vs Assignment

I am quite new to C++ and was wondering what is the difference(if any) between instantiating an object as

int main () {
  vector< int > x(2);
}

or

int main () {    
  vector< int > x = vector< int > (2); 
}

except for the fact that the latter takes longer to write. Thanks in advance!

解决方案

The difference is largely grammatical:

• vector<int> x(2); is direct initialization.

• vector<int> x = vector<int>(2); is copy initialization.

The latter formally requires that the class have an accessible copy constructor, but in practice the copy will be elided and the two versions produce exactly the same code.

You should always prefer direct initialization.

You can also go insane:

• vector<int> x = vector<int>(vector<int>(vector<int>(2)));

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