C++ Constructors vs Initialization Lists speed comparison - c++

Question

Are there any differences in execution time between constructors and initialization lists?(or is it just a matter of coding preference).
I have a set of objects that needs to be created frequently and would like to know if there is any performance gain by using initialization lists instead of constructors.
If I were to create a million instances of class A and another million of class B which choice would be better(the objects represent packets generated within a network hence these numbers).
class A {
private:
int a, b;
public:
A(int a_var, int b_var):a(a_var), b(b_var) {};
};
class B {
private:
int a, b;
public:
B(int a_var, int b_var) {
a = a_var;
b = b_var;
}
};
If any of the constructors is faster than the other for primitive types(as in the example) will it be faster if a and b were to be replaced by types?
Type example:
class AType {
private:
string a, b;
public:
AType(string a_var, string b_var):a(a_var), b(b_var) {};
};

Answer 1

The difference is for types with no trivial default constructor, which is called for you by compiler in your class B. Your class B is equivalent to:
class B {
private:
SleepyInt a, b;
public:
// takes at least 20s
B(int a_var, int b_var) : a(), b()
// ^^^^^^^^^^
{
a = a_var;
b = b_var;
}
};
If you do not place member variable or base class constructor in initialization list - ithe default constructor is called for them. int is basic type - its default constructor costs nothing - so no difference in your example, but for more complex types constructor+assignment might cost more than just constructing.
Some funny example, just to illustrate the difference:
class SleepyInt {
public:
SleepyInt () {
std::this_thread::sleep_for(std::chrono::milliseconds( 10000 ));
}
SleepyInt (int i) {}
SleepyInt & operator = (int i) { return *this; }
};
class A {
private:
SleepyInt a, b;
public:
A(int a_var, int b_var):a(a_var), b(b_var) {};
};
class B {
private:
SleepyInt a, b;
public:
// takes at least 20s
B(int a_var, int b_var) {
a = a_var;
b = b_var;
}
};

Answer 2

It is commonly accepted practice to use initialization lists as opposed to assignment in a constructor, and there's a very good reason for that.
Initialization lists can be used to initialize both POD (Plain Old Data) and user-defined types. When initializing a POD type, the effect is exactly the same as an assignment operator, meaning there is no performance difference between initialization lists or assignment in a constructor for POD types.
When we consider non-POD types things get more interesting. Before the constructor is called, constructors for the parent class and then any contained members are invoked, and by default the no-argument constructor is called. Using an initialization list you are able to choose which constructor is called.
So to answer the question, there is a performance difference, but only when initializing non-POD types.

Answer 3

If members are of more or less complex types, then the assignment initialization will first cause the default constructor call and then operator=, which may take longer.

Answer 4

There won't be a performance improvement if the types are built-in/intrinsic type.
That said:
Conclusion: All other things being equal, your code will run faster if
you use initialization lists rather than assignment.

Answer 5

Initialization list is benefical reference types, member class objects, or const members. Otherwise it takes more time.
Look at my test code:
#include <iostream>
#include <ctime>
using namespace std;
class A{
int a;
public:
A(int a_):a(a_){}
};
class B{
int b;
public:
B(){
}
B(int b_){
b=b_;
}
};
class C{
B b;
public:
C(int c_):b(c_){
}
};
class D{
B b;
public:
D(int d_){
b=d_;
}
};
int main()
{
clock_t start1[10], start2[10], end1[10], end2[10];
for(int j=0;j<10;j++){
start1[j]=clock();
for(int i=0;i<100000;i++){
A *newA=new A(i);
delete newA;
}
end1[j]=clock();
start2[j]=clock();
for(int i=0;i<100000;i++){
B *newB=new B(i);
delete newB;
}
end2[j]=clock();
}
double avg1=0, avg2=0;
for(int i=0;i<10;i++){
avg1+=(end1[i]-start1[i]);
avg2+=(end2[i]-start2[i]);
}
cout << avg1/avg2 << endl;
for(int j=0;j<10;j++){
start1[j]=clock();
for(int i=0;i<100000;i++){
C *newC=new C(i);
delete newC;
}
end1[j]=clock();
start2[j]=clock();
for(int i=0;i<100000;i++){
D *newD=new D(i);
delete newD;
}
end2[j]=clock();
}
avg1=avg2=0;
for(int i=0;i<10;i++){
avg1+=(end1[i]-start1[i]);
avg2+=(end2[i]-start2[i]);
}
cout << avg1/avg2 << endl;
system("pause");
return 0;
}
Example outputs like this:
1.02391
0.934741