A performance issue about copy constructor

These two day, I debugged a performance issue which is related to copy constructor: the class A has a member b which is NTL::ZZX type:

class A
{
    enum class type {zzx_t, ...} t;
    NTL::ZZX b;
    ......
}

When member t‘s value is zzx_t, b is valid. Otherwise b‘s content should be outdated.

There are 2 methods of implementing A‘s copy constructor:
(1)

A(const A& other) : t(other.t), b(other.b)
{
    ......
}

In this method, NTL::ZZX‘s copy constructor is called in spite of anything.

(2)

A(const A& other) : t(other.t)
{
    ......
    if (t == zzx_t)
    {
        b = other.b;
    }
    .....
}

In this case, NTL::ZZX‘s default constructor is called first. NTL::ZZX‘s copy assignment operator is invoked only if “t == zzx_t” condition is met.

NTL::ZZX‘s default constructor nearly does nothing, and copy constructor does approximate work as copy assignment operator. But in our scenario, t‘s value is not zzx_t at 80 percent of the time. So the second implementation of copy constructor gets a big performance boost compared to first one.