Can I create a std::vector of a struct without defining the struct first?

vector of structs c++ push_back
c++ struct
vector c++
vector in struct c++
c++ vector of struct pointers
c++ struct function
iterate through vector of structs c
c++ struct constructor

Instead of:

struct MyStruct{
    ...
};

std::vector<MyStruct> myVec;

Can I do something like:

std::vector<struct MyStruct {...}> myVec;

or even, since I don't need a name for this struct as I'm only using it inside this vector:

std::vector<struct {...}> myVec?

I'm using C++ for many years and I don't think it's possible. I think it would be nice though...

Or is there a way?

Last I checked, any type stored in a standard container (dunno why you reference STL, which is similar but different and in any case ambiguous!) must be complete. A (forwand-)declared type is not complete. If you want to know, please check the standard of the version you are using or at least mention that here.

Concerning the inline declaration of a struct in a template parameter, e.g. g++ says "types may not be defined in template arguments" when I try that. I can't give you a convincing reason why that shouldn't work. After all, all containers supply an element_type alias and with auto from C++ 11 that shouldn't be a problem in general.

struct (C programming language), as many members as desired, but the entire structure size must be known to the compiler. */ }; Always check that the vector is not empty & the index is valid while using the [] operator on std::vector. [] does not add elements if none exists, but it causes an Undefined Behavior if the index is invalid. You should create a temporary object of your structure, fill it up and then add it to the vector, using vector::push_back()

Both of the previous answers are correct - you cannot define a vector of something of incomplete type.

However, you can declare one - with the requirement that when you define it the type must be complete.

struct Foo; // forward declaration
using FooV = std::vector<Foo>; // declaration

struct Foo { int val; }; // definition
FooV v; // define a vector - all is well.

Return a `struct` from a function in C, b = a is that you didn't provide a complete type. A structure array is a data type that groups related data using data containers called fields.Each field can contain any type of data. Access data in a field using dot notation of the form structName.fieldName.

No, you cannot do that when you declare an actual variable or type, as C++ is a staticly typed language. The C++ construct to allow for general types are templates, i.e.

template<typename T>
using myVec = std::vector<T>;

which you can use directly, e.g.

struct someStruct { /* ... */ };
myVec<someStruct> foo;

or in some template code

template<typename T>
T bar(myVec<T> const&vec)
{
    /* ... */
}

Struct Constructor in C++?, , and the syntax is the same as for classes. Technically, a struct is like a class, so technically a struct would naturally benefit from having constructors and methods, like a class does. But this is only “technically” speaking. In practice, the convention is that we use struct s only to bundle data together, and a struct generally doesn’t have an interface with methods and everything.

Structs with Functions & Vectors in C++ Programming, This lesson will show you how to use structs with functions and vectors in C++. Multi Duration: 3:32 Posted: Jul 5, 2018 Defining Structures. In order to use members of a structure in a program, a structure must be defined first. The definition of a structure informs the C++ compiler about the type and the number of the data members forming the structure. The syntax for defining a structure is. struct structure_name

C++ structure: array of structures, pointer to structure, passing , Scope of variables · Pointers · Array · Std::array · Std::vector · String · Std::string · Pre-processor · Structure std::string name; We can also declare structure variables at the time of defining the structure as follows. cout << "First Student" << endl; cout << "roll no : " << p1.roll_no << endl; cout << "name : " << p1.name  In this article we will discuss different ways to initialize a vector in C++. Creating a vector object without any initialization will create an empty vector with no elements i.e. std::vector<int> vecOfInts;

S.4.7, Because structs are user-defined, we first have to tell the compiler what our In order to use the Employee struct, we simply declare a variable of type Employee: As with normal variables, struct member variables are not initialized, and will typically contain junk. std::cout << "Joe and Frank make the same amount\n";. Elements can be removed from a vector of vectors using the pop_back() function of C++ STL. Below example demonstrates the removal operation in a vector of vectors. The code removes elements from a 2D vector by using the pop_back() function and then displays the matrix.

Comments
  • You can't do that, unless you settle for a std::tuple.
  • You can't use them because they have no linkage (i.e., you can't find the ctor or dtor)
  • I'm not sure what you're trying to say, @Artyer.
  • When the template is instantiated, it gets the type as a symbol. structs declared in template parameters are unnamed, so they don't have a symbol name, so can't be used to instantiate a template (because the classes name incorporates the symbols passed to it, and also it can't use any members of the struct since it can't find them by name). They have "no linkage".
  • I don't see how it's relevant that C++ is a statically typed language.