C ++標(biāo)準(zhǔn)（第8.5節(jié)）說：如果程序要求對const限定類型T的對象進(jìn)行默認(rèn)初始化，則T必須是具有用戶提供的默認(rèn)構(gòu)造函數(shù)的類類型。為什么？我想不出在這種情況下為什么需要用戶提供的構(gòu)造函數(shù)的任何原因。struct B{  B():x(42){}  int doSomeStuff() const{return x;}  int x;};struct A{  A(){}//other than "because the standard says so", why is this line required?  B b;//not required for this example, just to illustrate      //how this situation isn't totally useless};int main(){  const A a;}