blocks|key|1434241|text|在C%2B%2B中，您可以重载ostream的operator<<和您的自定义类：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1434242|class+A+{
public:
++int+i;
};

std::ostream&+operator<<(std::ostream+&strm,+const+A+&a)+{
++return+strm+<<+"A("+<<+a.i+<<+")";
}|code-block|syntax|javascript|1434243|这样你就可以在streams上输出你的类的实例：|1434244|A+x+=+...;
std::cout+<<+x+<<+std::endl;|1434245|如果您的operator<<想要打印出类A的内部结构，并且确实需要访问它的私有和受保护的成员，您也可以将它声明为一个朋友函数：|1434246|class+A+{
private:
++friend+std::ostream&+operator<<(std::ostream&,+const+A&);
++int+j;
};

std::ostream&+operator<<(std::ostream+&strm,+const+A+&a)+{
++return+strm+<<+"A("+<<+a.j+<<+")";
}|1434247|entityMap^0|B|7|J|A|0|0|0|0|4|A|K|1|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|17|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|T|$]]

In C++ you can overload <code>operator&lt;&lt;</code> for <code>ostream</code> and your custom class:

<pre><code>class A {
public:
 int i;
};

std::ostream&amp; operator&lt;&lt;(std::ostream &amp;strm, const A &amp;a) {
 return strm &lt;&lt; "A(" &lt;&lt; a.i &lt;&lt; ")";
}
</code></pre>

This way you can output instances of your class on streams:

<pre><code>A x = ...;
std::cout &lt;&lt; x &lt;&lt; std::endl;
</code></pre>

In case your <code>operator&lt;&lt;</code> wants to print out internals of class <code>A</code> and really needs access to its private and protected members you could also declare it as a friend function:

<pre><code>class A {
private:
 friend std::ostream&amp; operator&lt;&lt;(std::ostream&amp;, const A&amp;);
 int j;
};

std::ostream&amp; operator&lt;&lt;(std::ostream &amp;strm, const A &amp;a) {
 return strm &lt;&lt; "A(" &lt;&lt; a.j &lt;&lt; ")";
}
</code></pre>

blocks|key|1219430|text|作为John所说的扩展，如果您想提取字符串表示并将其存储在std::string中，请执行以下操作：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1219431|#include+<sstream>++++
//+...
//+Suppose+a+class+A
A+a;
std::stringstream+sstream;
sstream+<<+a;
std::string+s+=+sstream.str();+//+or+you+could+use+sstream+>>+s+but+that+would+skip+out+whitespace|code-block|syntax|javascript|1219432|std::stringstream位于<sstream>报头中。|1219433|entityMap^0|T|B|0|0|0|H|J|9|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

As an extension to what John said, if you want to extract the string representation and store it in a <code>std::string</code> do this:

<pre><code>#include &lt;sstream&gt; 
// ...
// Suppose a class A
A a;
std::stringstream sstream;
sstream &lt;&lt; a;
std::string s = sstream.str(); // or you could use sstream &gt;&gt; s but that would skip out whitespace
</code></pre>

<code>std::stringstream</code> is located in the <code>&lt;sstream&gt;</code> header.

blocks|key|1219507|text|你也可以这样做，允许多态：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1219508|class+Base+{
public:
+++virtual+std::ostream&+dump(std::ostream&+o)+const+{
++++++return+o+<<+"Base:+"+<<+b+<<+";+";
+++}
private:
++int+b;
};

class+Derived+:+public+Base+{
public:
+++virtual+std::ostream&+dump(std::ostream&+o)+const+{
++++++return+o+<<+"Derived:+"+<<+d+<<+";+";
+++}
private:
+++int+d;
}

std::ostream&+operator<<(std::ostream&+o,+const+Base&+b)+{+return+b.dump(o);+}|code-block|syntax|javascript|1219509|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can also do it this way, allowing polymorphism:

<pre><code>class Base {
public:
 virtual std::ostream&amp; dump(std::ostream&amp; o) const {
 return o &lt;&lt; "Base: " &lt;&lt; b &lt;&lt; "; ";
 }
private:
 int b;
};

class Derived : public Base {
public:
 virtual std::ostream&amp; dump(std::ostream&amp; o) const {
 return o &lt;&lt; "Derived: " &lt;&lt; d &lt;&lt; "; ";
 }
private:
 int d;
}

std::ostream&amp; operator&lt;&lt;(std::ostream&amp; o, const Base&amp; b) { return b.dump(o); }
</code></pre>

blocks|key|1434461|text|在C%2B%2B11中，to_string最终被添加到标准中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1434462|http://en.cppreference.com/w/cpp/string/basic_string/to_string|offset|length|1434463|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|1Q|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

In C++11, to_string is finally added to the standard.

<a href="http://en.cppreference.com/w/cpp/string/basic_string/to_string" rel="noreferrer">http://en.cppreference.com/w/cpp/string/basic_string/to_string</a>

blocks|key|1434545|text|这个问题已经得到了回答。但我想添加一个具体的例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1434546|class+Point{

public:
++++++Point(int+theX,+int+theY)+:x(theX),+y(theY)
++++++{}
++++++//+Print+the+object
++++++friend+ostream&+operator+<<(ostream&+outputStream,+const+Point&+p);
private:
++++++int+x;
++++++int+y;
};

ostream&+operator+<<(ostream&+outputStream,+const+Point&+p){
+++++++int+posX+=+p.x;
+++++++int+posY+=+p.y;

+++++++outputStream+<<+"x="<<posX<<","<<"y="<<posY;
++++++return+outputStream;
}|code-block|syntax|javascript|1434547|这个例子需要理解操作符重载。|1434548|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The question has been answered. But I wanted to add a concrete example.

<pre><code>class Point{

public:
 Point(int theX, int theY) :x(theX), y(theY)
 {}
 // Print the object
 friend ostream&amp; operator &lt;&lt;(ostream&amp; outputStream, const Point&amp; p);
private:
 int x;
 int y;
};

ostream&amp; operator &lt;&lt;(ostream&amp; outputStream, const Point&amp; p){
 int posX = p.x;
 int posY = p.y;

 outputStream &lt;&lt; "x="&lt;&lt;posX&lt;&lt;","&lt;&lt;"y="&lt;&lt;posY;
 return outputStream;
}
</code></pre>

This example requires understanding operator overload.

I'd like to control what is written to a stream, i.e. <code>cout</code>, for an object of a custom class. Is that possible in C++? In Java you could override the <code>toString()</code> method for similar purpose.

C++ equivalent of Java's toString?

对于自定义类的对象，我希望控制写入流的内容，即cout。这在C++中是可能的吗？在Java语言中，您可以出于类似的目的覆盖toString()方法。

问Java的toString的C++等价物？
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