initializing-classes-and-structs-without-constructors-cpp.md

title	description	ms.date	ms.assetid
Brace initialization for classes, structs, and unions	Use brace initialization with any C++ class, struct, or union	09/28/2021	3e55c3d6-1c6b-4084-b9e5-221b151402f4

Brace initialization

It isn't always necessary to define a constructor for a class, especially ones that are relatively simple. Users can initialize objects of a class or struct by using uniform initialization, as shown in the following example:

// no_constructor.cpp
// Compile with: cl /EHsc no_constructor.cpp
#include <time.h>

// No constructor
struct TempData
{
    int StationId;
    time_t timeSet;
    double current;
    double maxTemp;
    double minTemp;
};

// Has a constructor
struct TempData2
{
    TempData2(double minimum, double maximum, double cur, int id, time_t t) :
       stationId{id}, timeSet{t}, current{cur}, maxTemp{maximum}, minTemp{minimum} {}
    int stationId;
    time_t timeSet;
    double current;
    double maxTemp;
    double minTemp;
};

int main()
{
    time_t time_to_set;

    // Member initialization (in order of declaration):
    TempData td{ 45978, time(&time_to_set), 28.9, 37.0, 16.7 };

    // When there's no constructor, an empty brace initializer does
    // value initialization = {0,0,0,0,0}
    TempData td_emptyInit{};

    // Uninitialized = if used, emits warning C4700 uninitialized local variable
    TempData td_noInit;

    // Member declaration (in order of ctor parameters)
    TempData2 td2{ 16.7, 37.0, 28.9, 45978, time(&time_to_set) };

    return 0;
}

When a class or struct has no constructor, you provide the list elements in the order that the members are declared in the class. If the class has a constructor, provide the elements in the order of the parameters. If a type has a default constructor, either implicitly or explicitly declared, you can use brace initialization with empty braces to invoke it. For example, the following class may be initialized by using both empty and non-empty brace initialization:

#include <string>
using namespace std;

class class_a {
public:
    class_a() {}
    class_a(string str) : m_string{ str } {}
    class_a(string str, double dbl) : m_string{ str }, m_double{ dbl } {}
double m_double;
string m_string;
};

int main()
{
    class_a c1{};
    class_a c1_1;

    class_a c2{ "ww" };
    class_a c2_1("xx");

    // order of parameters is the same as the constructor
    class_a c3{ "yy", 4.4 };
    class_a c3_1("zz", 5.5);
}

If a class has non-default constructors, the order in which class members appear in the brace initializer is the order in which the corresponding parameters appear in the constructor, not the order in which the members are declared (as with class_a in the previous example). Otherwise, if the type has no declared constructor, member initializers must appear in the brace initializer in the same order as they're declared. In this case, you can initialize as many of the public members as you wish, but you can't skip any member. The following example shows the order that's used in brace initialization when there's no declared constructor:

class class_d {
public:
    float m_float;
    string m_string;
    wchar_t m_char;
};

int main()
{
    class_d d1{};
    class_d d1{ 4.5 };
    class_d d2{ 4.5, "string" };
    class_d d3{ 4.5, "string", 'c' };

    class_d d4{ "string", 'c' }; // compiler error
    class_d d5{ "string", 'c', 2.0 }; // compiler error
}

If the default constructor is explicitly declared but marked as deleted, empty brace initialization can't be used:

class class_f {
public:
    class_f() = delete;
    class_f(string x): m_string { x } {}
    string m_string;
};
int main()
{
    class_f cf{ "hello" };
    class_f cf1{}; // compiler error C2280: attempting to reference a deleted function
}

You can use brace initialization anywhere you would typically do initialization—for example, as a function parameter or a return value, or with the new keyword:

class_d* cf = new class_d{4.5};
kr->add_d({ 4.5 });
return { 4.5 };

In /std:c++17 mode and later, the rules for empty brace initialization are slightly more restrictive. See Derived constructors and extended aggregate initialization.

initializer_list constructors

The initializer_list Class represents a list of objects of a specified type that can be used in a constructor, and in other contexts. You can construct an initializer_list by using brace initialization:

initializer_list<int> int_list{5, 6, 7};

Important

To use this class, you must include the <initializer_list> header.

An initializer_list can be copied. In this case, the members of the new list are references to the members of the original list:

initializer_list<int> ilist1{ 5, 6, 7 };
initializer_list<int> ilist2( ilist1 );
if (ilist1.begin() == ilist2.begin())
    cout << "yes" << endl; // expect "yes"

The standard library container classes, and also string, wstring, and regex, have initializer_list constructors. The following examples show how to do brace initialization with these constructors:

vector<int> v1{ 9, 10, 11 };
map<int, string> m1{ {1, "a"}, {2, "b"} };
string s{ 'a', 'b', 'c' };
regex rgx{ 'x', 'y', 'z' };

See also

Classes and Structs
Constructors