Are there completely-generic objects in C++ like in JavaScript?

Question

I'm new to C++, but with some experience in Javascript.

In JavaScript, when I write:

var object = {};

I create an object that is "completely generic" in the sense that it has no members/features, but it can have any member we like by adding that member later.

Can I create or simulate the this kind of an object in C++?

Answer 1

The answer is mostly-no, you can't.. In JavaScript, Object's can be likened to a dictionary or map from keys to typed values, with both the values and their types settable at run-time.

You can create a dictionary of arbitrary-typed values in C++, using the type-erasure capability of std::any, and strings for keys. With this approach, you can write:

using generic_object = std::unordered_map<std::string, std::any>
generic_object object;
object["foo"] = 123;
object["bar"] = [](char c) -> char { return c+1; };
object["baz"] = std::vector<int>{4,5,6,7};
std::any_cast<std::vector<int>>(object["bar"])[2] = 3 + std::any_cast<int>(object["foo"]);

You can't use plain-and-simple C++ syntax on new fields in your generic object, i.e. you can't write:

using generic_object = /* whatever */;
generic_object object;
object.foo = 123;                       // Compilation errors start here!
object.bar = []() -> char { return c+1; }
object.baz = std::vector<int>{4,5,6,7};
object.baz[2] = 3 + object.foo;
std::cout << "'a' becomes " << object.bar('a') << '\n';

like you may be used to from JavaScript. Instead, it would have to be:

struct {
   int foo;
   char (*bar)(char c);
   std::vector<int> baz;
} object;
object.foo = 123;
object.bar = []() -> char { return c+1; }
object.baz = std::vector<int>{4,5,6,7};
object.baz[2] = 3 + object.foo;
std::cout << "'a' becomes " << object.bar('a') << '\n';

And that's the "statically-typed" nature of C++. Or going even further, avoiding function pointers:

struct {
   int foo { 123 };
   char bar(char c) const { return c+1; }
   std::vector<int> baz {4,5,6,7};
} object;
object.baz[2] = 3 + object.foo;
std::cout << "'a' becomes " << object.bar('a') << '\n';

Use the clearly-typed approach, rather than the "dictionary-of-any's" I described above; that one would be cumbersome, slow, and frowned upon by most people.

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See also:

What is the difference between statically typed and dynamically typed languages?

Answer 2

JavaScript is dynamically typed whereas C++ is statically typed.

In JavaScript, as you showed in your example, you only need to write the variable name, without adding the type. After that, you can assign a string value but also an int value or a value of any other type.

Example:

var first;
var second;

In C++, you have to declare the type of the variable with the variable.

Example:

std::string str_value;
float float_value;
int int_value;