Predict the output of following C++ program.
The program calculates “2 raise to the power 8 (or 2^8)”. In fact, the structure funStruct can be used to calculate 2^n for any known n (or constant n). The special thing about above program is: calculation is done at compile time. So, it is compiler that calculates 2^8. To understand how compiler does this, let us consider the following facts about templates and enums:
1) We can pass nontype parameters (parameters that are not data types) to class/function templates.
2) Like other const expressions, values of enumaration constants are evaluated at compile time.
3) When compiler sees a new argument to a template, compiler creates a new instance of the template.
Let us take a closer look at the original program. When compiler sees funStruct<8>::val, it tries to create an instance of funStruct with parameter as 8, it turns out that funStruct<7> must also be created as enumaration constant val must be evaluated at compile time. For funStruct<7>, compiler need funStruct<6> and so on. Finally, compiler uses funStruct<1>::val and compile time recursion terminates. So, using templates, we can write programs that do computation at compile time, such programs are called template metaprograms. Template metaprogramming is in fact Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram. Template Metaprogramming is generally not used in practical programs, it is an interesting conecpt though.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.
- Template Specialization in C++
- Unordered Sets in C++ Standard Template Library
- The C++ Standard Template Library (STL)
- Pair in C++ Standard Template Library (STL)
- List in C++ Standard Template Library (STL)
- Deque in C++ Standard Template Library (STL)
- Priority Queue in C++ Standard Template Library (STL)
- Multiset in C++ Standard Template Library (STL)
- Map in C++ Standard Template Library (STL)
- Multimap in C++ Standard Template Library (STL)
- Class template with multiple parameters
- is_rvalue_reference Template in C++
- Containers in C++ STL (Standard Template Library)
- is_fundamental Template in C++
- is_pointer Template in C++
- generate_canonical Function template in C++
- std is_union() template in C++
- is_class template in C++
- std::is_enum Template in C++
- std::is_integral template in C++