Is there a way to initialise a new struct variable that does not involve writing a constructor?

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I think that I vaguely recall that one of the newer c++ standards (maybe its c++11, or maybe 14?...17??) allows you to initialise a struct, whereby you can define a struct and then initialise it without having to write a constructor.


struct test
    int a;
    int b;
    std::string str;

int main()
    std::map<int, test> test_map;
    test_map[0] = test(1, 2, "test1"); // This is the line in question
    // Or it might be more like: test_map[0] = test{1, 2, "test1"};
    return 0;

I can't recall the name of this special initialisation (or if it even exists)!. So my questions are:

  • Is there some new mechanism to achieve this without writing a constructor in the struct "test"?
  • If so, what is it called (so I can read more about it).

If this "feature" does not exist then please put me out of my misery!, it could be that my imagination has made this up...

The "initialize without a constructor" is called aggregate initialization, it has always been a part of C++ since day 1. Unfortunately, some might say.

In C++98 you could write:

std::map<int, test> test_map;
test temp = { 1, 2, "test1" };
test_map[0] = temp;

C++11 added that you can use aggregate initialization in prvalues, so you do not need to declare the intermediate variable (and there are no extra copies):

std::map<int, test> test_map;
test_map[0] = { 1, 2, "test1" };

std::map<int, test> m2 = { {0, {1, 2, "test2"}} };    // and this

C++ Structure Initialization, On the other hand, the C++11 does not have the designated initializers [See ISO/​IEC In order to achieve the same effect, use constructors or initializer lists: I know this question is quite old, but I found another way of initializing, using #​include <cstdio> struct Group { int x; int y; const char* s; }; int main() { Group group​  Write a C program to declare, initialize and access structures. In this post I will explain how to declare, initialize and access structures in C language. Different ways to initialize a structure variable and how to access structure members. Structure is a user defined data type. It is a collection of different types combined together to

Without a constructor, you can do it like this

test_map[0] = test{ 1, 2, "test1" };

or simply

test_map[0] = { 1, 2, "test1" };

Brace initialization for classes, structs, and unions, Use brace initialization with any C++ class, struct or union. no_constructor.cpp // Compile with: cl /EHsc no_constructor.cpp #include <time.h> // No constructor struct Member initialization (in order of declaration): TempData td{ 45978, The following examples show how to do brace initialization with  Because a structure-type variable can't be null (unless it's a variable of a nullable value type), you must instantiate an instance of the corresponding type. There are several ways to do that. Typically, you instantiate a structure type by calling an appropriate constructor with the new operator.

You can also default initialize:

struct test {
    int a{1};
    int b{2};
    std::string str{"test1"};

Or construct without assignment:

std::map<int, test> test_map{ 
    {0, {1, 2, "test1"}}

Or insert without copy:

    std::forward_as_tuple(1, 2, "test1"));

Should structs Have Constructors in C++, C++ structs, used as small bundles of data, are often depleted of any methods. be up to date with and learn something new from all fields of programming. Or are constructors not in the “spirit” of struct ? Or would constructors even get in the way? We can initialize an instance with the following syntax:. Is there some new mechanism to achieve this without writing a constructor in the struct "test"? If so, what is it called (so I can read more about it). If this "feature" does not exist then please put me out of my misery!, it could be that my imagination has made this up

Constructors and destructors, Constructors are a feature of C++ (but not C) that make initialization of structures convenient. Within a structure type definition, define a constructor in a way that is similar to a are written after new Cell, not after the name of the pointer variable p. can include more than one constructor, as long as no two constructors have​  Constructors for struct types resemble class constructors, but structs cannot contain an explicit parameterless constructor because one is provided automatically by the compiler. This constructor initializes each field in the struct to the default value.

Struct in C#, A structure is a value type that can contain constructors, constants, fields, be created with or without the new operator, same as primitive type variables. When you create a structure object without using new keyword, it does not call any constructor You can declare parameterized constructor to initialize struct members,  If you don't provide a constructor for your struct, C# relies on an implicit parameterless constructor to automatically initialize each field to its default value. For more information and examples, see Instance constructors. Constructor syntax. A constructor is a method whose name is the same as the name of its type.

Constructors and member initializer lists, 3 Initialization order; 4 Example Constructors are declared using member function declarators of the may include the member initializer list, whose syntax is the colon struct S { int n; S(int); // constructor declaration S() : n(7) Member initializer list is the place where non-default initialization of these  The advantage of NOT writing a constructor If a structure is reduced to its bare minium, with no constructor, no method, no inheritance, no private method or data, no member initializer, if a structure only defines public data members, then an special initialization feature of C++ kick in: aggregate initialization.