Question

只有在被宣布为初始目标时,该目标才可以适用:

struct C { /* ... */ };
C f();
 
struct D;
D g();
 
struct D : C
{
    D() : C(f()) {}    // no elision when initializing a base-class subobject
    D(int) : D(g()) {} // no elision because the D object being initialized might
                       // be a base-class subobject of some other class
};

If the compiler can make out the memory layout of a non-overlapping object (such as of a primary data type) to elide copy and instead construct the object at its destination, why can t it do the same for overlapping objects as well? What is the limitation at the system level that would make it not possible for the compiler to do such copy elision?

Answer 1

why can t it do the same for overlapping objects as well?

由于标的大小是错误的。案件

D() : C(f()) {}

the sizeof(D) can be larger than sizeof(C) and if it is then you can t elide the base class as then there is no room for the derived part in the elided base object.

With

D(int) : D(g()) {}

You get the same thing in the other direction. If D is a base class of another object then eliding D(g()) would mean that the object that actually gets created is a D and not whatever inherited it and if it is larger than you are in trouble.

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