What are primitive types default-initialized to in C++?

user_57428 0
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When I use an initialization list:

struct Struct {
    Struct() : memberVariable() {}
    int memberVariable;
};

the primitive type (int, bool, float, enum, pointer) member variable is default-initialied. Is the value it gets implementation defined or is it the same for all implementations?

initialization c++
7 Answers

Default initialization of non-class variables with automatic and dynamic storage duration produces objects with indeterminate values (static and thread-local objects get zero initialized)

References and const scalar objects cannot be default-initialized. see below example

#include <string>
 
struct T1 { int mem; };
 
struct T2
{
    int mem;
    T2() { } // "mem" is not in the initializer list
};
 
int n; // static non-class, a two-phase initialization is done:
       // 1) zero initialization initializes n to zero
       // 2) default initialization does nothing, leaving n being zero
 
int main()
{
    int n;            // non-class, the value is indeterminate
    std::string s;    // class, calls default ctor, the value is "" (empty string)
    std::string a[2]; // array, default-initializes the elements, the value is {"", ""}
//  int& r;           // error: a reference
//  const int n;      // error: a const non-class
//  const T1 t1;      // error: const class with implicit default ctor
    T1 t1;            // class, calls implicit default ctor
    const T2 t2;      // const class, calls the user-provided default ctor
                      // t2.mem is default-initialized (to indeterminate value)
}
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