blocks|key|3328362|text|它防止someMethod更改该类对象的任何成员变量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3328363|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|$]]

It prevents someMethod from altering any member variable of an object of that class.

blocks|key|343008|text|它告诉编译器该方法对类状态没有影响；您不能对其中的任何内容赋值。看一看C%2B%2B+FAQ+Lite+18.10。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|343009|entityMap|0|LINK|mutability|MUTABLE|url|http://www.parashift.com/c%2B%2B-faq-lite/const-correctness.html^0|Z|I|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

It tells the compiler that the method has no effect on the classes state; you can't assign to anything in it. Have a look at the <a href="http://www.parashift.com/c++-faq-lite/const-correctness.html" rel="noreferrer">C++ FAQ Lite 18.10</a>.

blocks|key|3328397|text|它允许您在const对象上调用类成员函数：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3328398|class+SomeClass
{
public:
++++void+foo();
++++void+bar()+const;
}

SomeClass+a;
const+SomeClass+b;

a.foo();++//+ok
a.bar();++//+ok
b.foo();++//+ERROR+--+foo()+is+not+const
b.bar();++//+ok+--+bar()+is+const|code-block|syntax|javascript|3328399|还有用于volatile对象的volatile限定符，也可以将用于const+volatile对象的函数设为const+volatile，但这两种方法非常少见。|3328400|entityMap^0|5|5|0|0|4|8|F|8|X|E|1I|E|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]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|N|$]]

It allows you to call the class member function on <code>const</code> objects:

<pre><code>class SomeClass
{
public:
 void foo();
 void bar() const;
}

SomeClass a;
const SomeClass b;

a.foo(); // ok
a.bar(); // ok
b.foo(); // ERROR -- foo() is not const
b.bar(); // ok -- bar() is const
</code></pre>

There's also the <code>volatile</code> qualifier for use with <code>volatile</code> objects, and you can also make functions <code>const volatile</code> for use on <code>const volatile</code> objects, but those two are exceedingly rare.

blocks|key|3328436|text|我关于优化的第一个想法是，因为'const‘表示实例的状态没有改变，所以编译器可能有更多的自由来重新排序对该实例上的方法的附近调用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3328437|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|$]]

My first thought regarding optimization is that since the 'const' indicates that the instance's state hasn't changed, the compiler possibly has more freedom with respect to reordering nearby calls to methods on that instance.

blocks|key|346468|text|无论const是否存在，为const方法生成的asm代码都是相同的。const是编译器而不是运行时的函数，所以如果有任何性能提升，我认为编译器优化器可能会使用const作为提示，比如内联或确定可能优化的副作用。因此，简而言之，优化器可能会提供一些帮助，但如果方法一开始就是直接的，那么我怀疑优化器生成的代码是否会是任何不同的常量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|346469|这是我使用的一个简单的优化(而不是像const这样的碰碰撞撞的事情)，它只需一秒钟就能得到回报。组织类变量，使它们更好地落在缓存线边界上，并将最常访问的变量放在一起。要做到这一点，只需将整型、双精度型、浮点型等放在类变量声明的顶部，将奇数大小的变量放在底部，如下所示：|346470|int+foo;+
int+bar;
double+baz;
SomeObject+obj;
char+ch[14];|code-block|syntax|javascript|346471|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The asm code that is generated for the const method will be the same if the const is there or not. const is a function of the compiler not the runtime, so if there are any performance gains I would think that the compilers optimizer might use the const as a hint for things like inlining or determining side effects for a possible optimization. So in short the optimizer might be able to help out a bit, but if the method is straight forward to begin with then I doubt that the code generated from the optimizer would be any different const or no const.

Here's an easy optimization I use (rather than hit and miss things like const) which take a second but pay off. Organize your class variables so that they fall on cache line boundaries a little better, and put your most accessed variables together. To do it just put your ints, doubles, floats, etc. together at the top of your class variable declarations and your odd sized variables at the bottom like so:

<pre><code>int foo; 
int bar;
double baz;
SomeObject obj;
char ch[14];
</code></pre>

blocks|key|346493|text|如果编译器知道类实例的字段在const成员函数调用中没有被修改，它就不必重新加载在const函数调用之前可能已经保存在寄存器中的任何字段。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|346494|这在某种程度上参考了discussion+on+const_cast中的C%2B%2B常见问题。|offset|length|346495|entityMap|0|LINK|mutability|MUTABLE|url|http://www.parashift.com/c%2B%2B-faq-lite/const-correctness.html#faq-18.14^0|0|A|O|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

If the compiler knows that the fields of a class instance are not modified across a const member function call, it doesn't have to reload any fields that it may have kept in registers before the const function call.

This is sort of referred to the in C++ FAQ in the <a href="http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.14" rel="noreferrer">discussion on const_cast</a>.

I've seen some methods like this:

<pre><code>void SomeClass::someMethod() const;
</code></pre>

What does this const declaration do, and how can it help optimize a program?

Edit

I see that the first part of this question has been asked before... BUT, it still doesn't answer the second part: how would this optimize the program?

How does const after a function optimize the program?

我见过一些这样的方法：void SomeClass::someMethod() const;这个const声明做了什么，它如何帮助优化程序？编辑我看到这个问题的第一部分以前已经被问过了。但是，它仍然没有回答第二部分：，这将如何优化程序？

问函数后的const如何优化程序？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问函数后的const如何优化程序？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问函数后的const如何优化程序？
EN