C++ is a highly evolved and mature programming language. The C++ language has a great set of choices of modern C++ tool sets and compilers all of which come with a lot of utilities, GUI components, and libraries. C++11 was a big step for functional programming with C++. It has brought many other features like automatic type deduction (auto), lambda expressions, and decltype features. In this post, we explain what decltype is and how to use it with today’s modern C++.
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What is the decltype type specifier in C++?
The decltype keyword and operator represents the type of a given entity or expression. This feature is one of the C++11 features added to compilers (including BCC32 and other CLANG compilers). In a way you are saying “I am declaring this variable to be the same type as this other variable“.
Here is the Syntax:
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decltype ( expression or entity ) |
Note that, C++11 brings us the C++11 Standard Library and Threading Library with a lot of new features like Lambda expressions, automatic type deduction (auto) and decltype(e), uniform initialization syntax (mapping etc.), deleted and defaulted functions, uniform initialization syntax, Rvalue References, new smart pointer classes, and new C++ algorithms <algorithm>. Most of these features enable C++ to be a functional programming language. Typical C++ functions for use in functional programming include: map, filter, and reduce. There are also the functions std::transform, std::remove_if, and std::accumulate.
Lambdas are one of the great additions to C++11 and has changed the way we write C++ code. In general, most programmers argue that modern C++ code should be using lambdas because it allows us to create in-place anonymous functions which in turn makes C++ a Functional Programming Language.
Is there a simple example of how to use the decltype type specifier in C++?
It’s easier to explain decltype with an example. Let’s assume there is a variable c and you don’t know its type. You can use decltype. Here is a simple example.
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char c; decltype(c) c2; // char c2; |
here is more detailed example:
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char c='A'; int i = 10; float f = 9.81; std::string s = "this is string"; decltype(c) c2; // char c2; decltype(i) i2; // int i2; decltype(f) f2; // float f2; decltype(s) s2; // string s2; |
in this example only types of new variables are the same as the old ones. Values of new variables are their default values, values are not copied. In other words it is only used to declare types of that given type.
Is there rules about decltype type specifier in C++ ?
Here are the rules for evaluating decltype.
- If
eis an identifier expression or is accessing a class member,decltype(e)is the type of the identifier or accessor designated bye. If there is no such thing, or ifeis the name of a set of overloaded functions, so there is ambiguity,decltype(e)is invalid. - Otherwise, if
eis a function call or invokes an overloaded operator,decltype(e)is the type returned by the function. - Otherwise, if
eis an lvalue,decltype(e)is a reference toT (T&), where T is the type ofe. - If none of the other cases apply,
decltype(e)is the type ofe.
Here e is a variable that it can be a char, int, float, double, long long int, string, a member of a struct or class object, etc.
Is there a full example of how to use the decltype type specifier in C++?
Here is a simple example that uses decltype to declare new variables.
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#include <iostream> #include <string> int main() { char c='A'; int i = 10; float f = 9.81; std::string s = "this is string"; std::cout << c << "," << i << "," << f << "," << s << "," << std::endl; // lets use decltype to declare new variables decltype(c) c2; // char c2; decltype(i) i2; // int i2; decltype(f) f2; // float f2; decltype(s) s2; // string s2; std::cout << c2 << "," << i2 << "," << f2 << "," << s2 << "," << std::endl; //lets use our new variables c2 = c + 1; i2 = i + 1; f2 = f + 0.02; s2 = s + " two"; std::cout << c2 << "," << i2 << "," << f2 << "," << s2 << "," << std::endl; system("pause"); return 0; } |
Here is another full example that uses decltype to define members of other types like functions, structs, classes, etc,
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#include <iostream> const int* foo() { return new int[0]; } struct A { double value; }; class B { int value; public: const A* function() { return new A(); } }; double GetValue(int one); long int GetValue(double d); template<class T> class C { public: T* value; }; int main() { double e; const char *pch; char ch; A* a = new A(); B* b = new B(); C<B> *c = new C<B>(); decltype(pch) var1; // type is const char* decltype(ch) var2; // type is char decltype(a) var4; // type is A* decltype(a->value) var5; // type is double decltype((a->value)) var6 = e; // type is const double& decltype(foo()) var7; // f is const int* decltype(b->function()) var8; // type is const A* decltype(c->value) var9; // type is B* decltype(GetValue(e)) var10; // well-formed, the declaration is not } |
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