c++ - "A()" vs. "A() = default;" vs. Implicit A()? -

#include <vector>  // version 1 struct {     std::vector<int> m_coll;      // compiler generate ctor a() here };  // version 2 struct {     std::vector<int> m_coll;     a(){} };  // version 3 struct {     std::vector<int> m_coll;     a() : m_coll(){} };  // version 4 struct {     std::vector<int> m_coll;     a() = default; }; 

what differences between 4 versions?

is m_coll guaranteed default-initialized in of these versions?

it easier consider various options whenever add datum. consider:

struct {   int i;   std::vector<int> coll; }; 

in case, compiler generates default c'tor you, i uninitialized, you'll have set explicitly.

let's improve things:

struct b {   int {};   std::vector<int> coll; }; 

for b, compiler generates default c'tor you, i initialized in-class, , default-constructed object of type b initialized. suppose want add user-defined c'tor:

struct c {   int {};   std::vector<int> coll;   c(int const j) : i{j} {} }; 

adding user-defined c'tor suppresses automatic generation of default constructor. enable default c'tor, 1 can few different things:

struct d1 {   int {};   std::vector<int> coll;   d1(int const j) : i{j} {}   d1(){} }; 

although above well-formed, find ugly. here d1(){} is default c'tor, , i appropriately initialized since has in-class initializer. however, more descriptive like:

struct d2 {   int {};   std::vector<int> coll;   d2(int const j) : i{j} {}   d2() = default; }; 

this way, can read enabling default c'tor.

in experience, more helpful use default whenever 1 needs enable default definition copy/move constructors , assignment operators , default definition virtual destructors.

long story short: use default whenever need enable c'tor or assignment operator compiler otherwise suppress, or when need default definition virtual destructor.