blocks|key|4409034|text|Car+buyCar();没有人们认为的效果，查一下“最烦人的语法分析”：https://en.wikipedia.org/wiki/Most_vexing_parse|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4409035|在函数或方法的上下文中：|4409036|由于C%2B%2B语法中的怪癖，语法Type+instance()实际上被解释为声明而不是调用。请参阅calling+the+default+constructor|4409037|若要调用某个类型的默认无参数构造函数，需要省略括号。|4409038|Car+buyCar;+//+allocates+and+constructs+a+`Car`+on+the+stack|code-block|syntax|javascript|4409039|然而，如果你是在堆上分配(使用new)，那么使用括号是可行的。|4409040|使用原始指针：|4409041|Car*+buyCar+=+new+Car();|4409042|使用智能指针：|4409043|unique_ptr<Car>+buyCar+=+make_unique<Car>();+//+<--+parens+used+because+this+is+actually+calling+`make_unique`+(a+function)+which+indirectly+calls+the+constructor|4409044|在类型定义(即字段)中：|4409045|在初始化列表中，您确实使用括号来调用字段的默认构造函数-但是这在很大程度上是没有意义的，因为类型的默认(编译生成)构造函数已经这样做了：|4409046|class+Inner+{
public:
++++Inner()+{
++++}
}

class+Container+{
private:
++++Inner+innerInstance;
public:
++++Container()+:
++++++++innerInstance()+//+<--+parens+used+here
++++{+}
}|4409047|entityMap|0|LINK|mutability|MUTABLE|url|https://en.wikipedia.org/wiki/Most_vexing_parse|1|https://stackoverflow.com/questions/5300124/calling-the-default-constructor^0|0|D|11|1B|0|0|0|E|F|1B|V|1|0|0|0|F|3|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@$9|1H|A|1I|B|C]]|D|@$9|1J|A|1K|1|1L]]|E|$]]|$1|F|3|G|5|6|7|1M|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1N|8|@$9|1O|A|1P|B|C]]|D|@$9|1Q|A|1R|1|1S]]|E|$]]|$1|J|3|K|5|6|7|1T|8|@]|D|@]|E|$]]|$1|L|3|M|5|N|7|1U|8|@]|D|@]|E|$O|P]]|$1|Q|3|R|5|6|7|1V|8|@$9|1W|A|1X|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1Y|8|@]|D|@]|E|$]]|$1|U|3|V|5|N|7|1Z|8|@]|D|@]|E|$O|P]]|$1|W|3|X|5|6|7|20|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|N|7|21|8|@]|D|@]|E|$O|P]]|$1|10|3|11|5|6|7|22|8|@]|D|@]|E|$]]|$1|12|3|13|5|6|7|23|8|@]|D|@]|E|$]]|$1|14|3|15|5|N|7|24|8|@]|D|@]|E|$O|P]]|$1|16|3|-4|5|6|7|25|8|@]|D|@]|E|$]]]|17|$18|$5|19|1A|1B|E|$1C|1D]]|1E|$5|19|1A|1B|E|$1C|1F]]]]

<code>Car buyCar();</code> does not have the effect people think, look up "The Most Vexing Parse": <a href="https://en.wikipedia.org/wiki/Most_vexing_parse" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Most_vexing_parse</a>

<h3>In the context of a function or method:</h3>

Due to quirks in C++'s grammar, the syntax <code>Type instance()</code> is actually interpreted as a declaration instead of as an invocation. See <a href="https://stackoverflow.com/questions/5300124/calling-the-default-constructor">calling the default constructor</a>

To call the default, parameterless, constructor of a type you need to omit the parenthesis.

<pre><code>Car buyCar; // allocates and constructs a `Car` on the stack
</code></pre>

However, if you're allocating on the heap (with <code>new</code>), then using the parenthesis does work.

Using raw pointers:

<pre><code>Car* buyCar = new Car();
</code></pre>

Using smart pointers:

<pre><code>unique_ptr&lt;Car&gt; buyCar = make_unique&lt;Car&gt;(); // &lt;-- parens used because this is actually calling `make_unique` (a function) which indirectly calls the constructor
</code></pre>

<h3>In a type definition (i.e. a field):</h3>

In an initialization list, then you do use parenthesis to call the default constructor of a field - however this is largely pointless as the default (compile-generated) constructor for the type will already do this:

<pre><code>class Inner {
public:
 Inner() {
 }
}

class Container {
private:
 Inner innerInstance;
public:
 Container() :
 innerInstance() // &lt;-- parens used here
 { }
}
</code></pre>

blocks|key|2092639|text|此方法应返回Car类的对象。默认构造函数是自动可用的，它不需要单独定义。也可以创建自定义构造函数，并使用所需的值进行初始化。因此，您可以根据需求返回默认构造函数或自定义构造函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2092640|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

This method should return the object of the Car class. Default constructor is automatically available, it needs not to be defined separately. Custom constructor can also be created and initialized with required values. So, either you can return the default constructor or custom constructor based upon the requirements.

blocks|key|2092669|text|我已经为你写了一个小例子|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2092670|#include+<iostream>
using+namespace+std;
class+Car+{
public+:
++++Car():+carnum(0)+{
++++++++cout+<<+"default+constructor+"+<<+carnum+<<+endl;
++++}

++++Car+(int+n)+:+carnum+(n)+{
++++++++cout+<<+"Argument+constructor+"+<<+carnum+<<+endl;+
++++}

++++Car&+operator+=+(const+Car&+car)+{

++++++++carnum+=+car.carnum;
++++++++cout+<<+"operator+=+"+<<+carnum+<<+endl;
++++++++return+*this;
++++}

++++Car+(const+Car&+car)+{
++++++++carnum+=+car.carnum;
++++++++cout+<<+"Copy+constructor+"+<<+carnum+<<+endl;
++++}

++++Car+buyCar()+{
++++++++return+*this;
++++}

++++int+carnum;
};
int+main()+{
++++Car+aCar,+theCar(2);+//+default+constructor+for+aCar+and+argument+constructor+for+theCar;
++++cout+<<+endl;
++++aCar+=+theCar.buyCar();+//+copy+constructor+and+equal+operator+are+called
++++cout+<<+endl;
++++Car+bCar(aCar.buyCar());+//+only+copy+constructor+is+called+here
++++cout+<<+endl;
++++return+0;
}|code-block|syntax|javascript|2092671|因此，这取决于您如何使用buyCar()函数的上下文。|offset|length|style|CODE|2092672|我假设buyCar()是class+Car的成员函数|2092673|Example|2092674|entityMap|0|LINK|mutability|MUTABLE|url|http://ideone.com/XdQFh4^0|0|0|C|8|0|3|8|C|9|0|0|7|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@$I|14|J|15|K|L]|$I|16|J|17|K|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|18|8|@]|9|@$I|19|J|1A|1|1B]]|A|$]]|$1|Q|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

I have written a small example for you

<pre><code>#include &lt;iostream&gt;
using namespace std;
class Car {
public :
 Car(): carnum(0) {
 cout &lt;&lt; "default constructor " &lt;&lt; carnum &lt;&lt; endl;
 }

 Car (int n) : carnum (n) {
 cout &lt;&lt; "Argument constructor " &lt;&lt; carnum &lt;&lt; endl; 
 }

 Car&amp; operator = (const Car&amp; car) {

 carnum = car.carnum;
 cout &lt;&lt; "operator = " &lt;&lt; carnum &lt;&lt; endl;
 return *this;
 }

 Car (const Car&amp; car) {
 carnum = car.carnum;
 cout &lt;&lt; "Copy constructor " &lt;&lt; carnum &lt;&lt; endl;
 }

 Car buyCar() {
 return *this;
 }

 int carnum;
};
int main() {
 Car aCar, theCar(2); // default constructor for aCar and argument constructor for theCar;
 cout &lt;&lt; endl;
 aCar = theCar.buyCar(); // copy constructor and equal operator are called
 cout &lt;&lt; endl;
 Car bCar(aCar.buyCar()); // only copy constructor is called here
 cout &lt;&lt; endl;
 return 0;
}
</code></pre>

So, it depends on the context how you are using <code>buyCar()</code> function.

I have assumed that <code>buyCar()</code> is a member function of <code>class Car</code>

<a href="http://ideone.com/XdQFh4" rel="nofollow noreferrer">Example</a>

blocks|key|4409159|text|即使class+Car没有Constructors和Assignment+Operator，函数buyCar()仍然可以使用默认构造函数和赋值操作符。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4409160|但是如果您的类处理动态内存，那么如果类有一些动态创建/分配的数据成员，那么该类应该有Copy+Constructor、Move+Constructor、Assignment+Operator和Move+Assignment+operator|4409161|entityMap^0|2|9|D|C|Q|J|1C|8|0|16|G|1N|G|24|J|2O|O|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|I|$]]

Even if <code>class Car</code> do not have <code>Constructors</code> and <code>Assignment Operator</code>, the function <code>buyCar()</code> would still work with default constructors and assignment operators.

But if your class deals with dynamic memory, that if the class has some data members which are dynamically created/allocated then the class should have the <code>Copy Constructor</code>, <code>Move Constructor</code>, <code>Assignment Operator</code> and <code>Move Assignment operator</code>

suppose we have a function like below

<pre><code>Car buyCar();
</code></pre>

My question is which method we must have in Car class for this function to work ?
Is it default constructor ?

returning object from class function c++

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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假设我们有一个如下的函数Car buyCar();我的问题是，我们必须在Car类中有哪个方法才能让这个函数工作？它是默认构造函数吗？

问从类函数c++返回对象
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问从类函数c++返回对象EN