c++中的trivial

一、什么是trivial

说到TrivialType就得提到c++中的TrivialType类型？在c++中有两个要求：
第一，可平凡复制 (TriviallyCopyable)
第二，若该类型是类类型或其数组，则该类拥有一个或多个合格的默认构造函数，均为平凡的。
更进一步表述在std::is_trivial中，它是如下说明的：
“标量类型、有平凡默认构造函数的可平凡复制类，或这些类/类型的数组，可有 cv 限定”
看一下具体的英文表述：

Trivial default constructorThe default constructor for class T is trivial (i.e. performs no action) if all of the following is true:The constructor is not user-provided (i.e., is implicitly-defined or defaulted)
T has no virtual member functions
T has no virtual base classes
T has no non-static members with default initializers.(since C++11)
Every direct base of T has a trivial default constructor
Every non-static member of class type has a trivial default constructor
A trivial default constructor is a constructor that performs no action. Objects with trivial default constructors can be created by using reinterpret_cast on any suitably aligned storage, e.g. on memory allocated with std::malloc. All data types compatible with the C language (POD types) are trivially default-constructible.Trivial copy constructorThe copy constructor for class T is trivial if all of the following is true:it is not user-provided (that is, it is implicitly-defined or defaulted), and if it is defaulted, its signature is the same as implicitly-defined (until C++14);
T has no virtual member functions;
T has no virtual base classes;
the copy constructor selected for every direct base of T is trivial;
the copy constructor selected for every non-static class type (or array of class type) member of T is trivial;
T has no non-static data members of volatile-qualified type.(since C++14)
A trivial copy constructor is a constructor that creates a bytewise copy of the object representation of the argument, and performs no other action. Objects with trivial copy constructors can be copied by copying their object representations manually, e.g. with std::memmove. All data types compatible with the C language (POD types) are trivially copyable.Trivial destructorThe destructor for class T is trivial if all of the following is true:The destructor is not user-provided (meaning, it is either implicitly declared, or explicitly defined as defaulted on its first declaration)
The destructor is not virtual (that is, the base class destructor is not virtual)
All direct base classes have trivial destructors
All non-static data members of class type (or array of class type) have trivial destructors
A trivial destructor is a destructor that performs no action. Objects with trivial destructors don't require a delete-expression and may be disposed of by simply deallocating their storage. All data types compatible with the C language (POD types) are trivially destructible.

那么，trivial到底是什么意思呢？从前面也可以隐约猜测出来，英文原意它是“细小的，不重要的”意思，在c++一般译作平凡的，有平凡的当然就有不平凡的，不符合平凡的条件的都是不平凡的。

二、作用

那么问题来了，c++中搞这个trivial和non-trivial干啥？说白了你看这些定义就明白了，c++中如果有用户自定义的ctor/dtor，那按照c++的标准，就必须调用这些用户自定义的相关函数，那么其中的是否会有一些没有效率的代码，标准是不管的。但是如果编译器能够判断出trivial，那么它就可以直接按照自己觉得怎么快怎么好用来编译处理这些代码，反正trivial下这些代码都是没啥意义的。这样一来，就会在编译阶段进行一轮代码的优化。比如拷贝两个类对象，如果有自定义的拷贝函数，那么就会调用自定义的，否则就可以直接内存拷贝（memcpy）。

三、例程

看一下具体的例子：

#include <iostream>
#include <type_traits>struct A {int m;
};struct B {B() {}
};int main()
{std::cout << std::boolalpha;std::cout << std::is_trivial<A>::value << '\n';std::cout << std::is_trivial<B>::value << '\n';
}

再看一个判断平凡复制的例子：

#include <iostream>
#include <type_traits>struct A {int m;
};struct B {B(const B&) {}
};struct C {virtual void foo();
};int main()
{std::cout << std::boolalpha;std::cout << std::is_trivially_copyable<A>::value << '\n';std::cout << std::is_trivially_copyable<B>::value << '\n';std::cout << std::is_trivially_copyable<C>::value << '\n';
}

例子非常简单，没有什么可解释的。

三、总结

从这一点来看，所有的相关关键点，都需要从背后的设计出发，单纯的理解一个关键字一个函数，往往陷入细节的纠缠中去。有的时候儿，从细节中跳出来，发现问题的根本，就更容易理解这个问题。如果正向推一个问题比较麻烦，可以试着从反向推一下，再不行，从第三方的角度推一下。
方法论很重要。

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