033 - 问，什么是trivial classes？

033 - 问，什么是trivial classes？

Trivial is the first property mentioned above: trivial classes support static initialization. If a class is trivially copyable (a superset of trivial classes), it is ok to copy its representation over the place with things likememcpy and expect the result to be the same.

例子（c++11）

// empty classes are trivial
struct Trivial1 {};
// all special members are implicit
struct Trivial2 {
int x;
};
struct Trivial3 : Trivial2 { // base class is trivial
Trivial3() = default; // not a user-provided ctor
int y;
};
struct Trivial4 {
public:
int a;
private: // no restrictions on access modifiers
int b;
};
struct Trivial5 {
Trivial1 a;
Trivial2 b;
Trivial3 c;
Trivial4 d;
};
struct Trivial6 {
Trivial2 a[23];
};
struct Trivial7 {
Trivial6 c;
void f(); // it's okay to have non-virtual functions
};
struct Trivial8 {
int x;
static NonTrivial1 y; // no restrictions on static members
}
struct Trivial9 {
Trivial9() = default; // not user-provided
// a regular constructor is okay because we still have default ctor
Trivial9(int x) : x(x) {};
int x;
}
struct NonTrivial1 : Trivial 3 {
virtual f(); // virtual members make non-trivial ctors
}
struct NonTrivial2 {
NonTrivial2() : z(42) {} // user-provided ctor
int z;
}
struct NonTrivial3 {
NonTrivial3(); // user-provided ctor
int w;
}
NonTrivial3::NonTrivial3() = default; // defaulted but not on first declaration
// still counts as user-provided
struct NonTrivial5 {
virtual ~NonTrivial5(); // virtual destructors are not trivial
};

参考资料

http://stackoverflow.com/questions/4178175/what-are-aggregates-and-pods-and-how-why-are-they-special/4178176#4178176

http://stackoverflow.com/questions/3899223/what-is-a-non-trivial-constructor-in-c