Usage of brace-enclosed initializer list in struct and class in C++
We know struct is a constructor in C language while class is based on struct in C++. However, that doesn’t mean struct is discarded in C++, inversely, it has been given some new characters in C++, (e.g.:struct can be inherited from struct or class, we can declare public or private variable and function in struct , we can declare constructor and destructor in struct, we can also declare template for struct(compile polymorphism), …). Based on my experience, the only difference between struct and class is in class we can declare virtual function(runtime polymorphism) while in struct we can’t. OK, it’s not today’s point. Today we discuss about the {}. To make explanation be concise Let’s see sample below:
Sample Code1
#include <iostream>
using namespace std;
struct A{
public:
int number = 1;
const char* symbol = "b";
};
int main(){
A pp = {2, "a"};
//Tip: << is an overloaded operator which returns an object so that we can use <<A<<B<< ... <<C<<D sequencely.
cout << pp.number << endl << pp.symbol << endl;
}
Output1
- pp.number:2
- pp.symbol:a
Sample Code2
#include <iostream>
using namespace std;
struct A{
public:
int number = 1;
private:
const char* symbol = "b";
};
int main(){
A pp = {2, "a"};
cout << pp.number << endl << pp.symbol << endl;
}
Output2
- error: could not convert ‘{2, “a”}’ from ‘
' to 'A' - error: ‘const char*A::symbol’ is private within this context
- note: declared private here
From results above, we know variable in struct can be initialized by {} but only public variable can be.Let’s modify struct A:
Sample Code3
#include <iostream>
using namespace std;
class A{
public:
A(){};
virtual ~A(){};
int number = 1;
const char* symbol = "b";
};
int main(){
A pp = {2, "a"};
cout << pp.number << endl << pp.symbol << endl;
}
Output3
- error: could not convert ‘{2, “a”}’ from ‘
' to 'A'
From result above, we know if we has defined a constructor in struct(or class), {} can not be used because C++ complier will force us to initialize varible in the constructor.
Conclusion: {} can be used in both struct and class, but only can be used to initialize public variable. In addition, if we have declared a constructor, we can’t use {}. Sometimes it’s so simple and convinient, right?
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