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c++templatesinstantiation

Why is the size of a template 1 Byte in C++?

With using, an alias for a type can be defined and a unit of this type has the same size like an original instantation (1). A template however only weights 1 byte (2).

using aliasOfInt = int;

template<typename T>
struct intWrapper {
    using wrappedAliasOfInt = T;
};


int main() {
    // (1) --> 4B
    std::cout << "sizeof(aliasOfInt)      = " << sizeof(aliasOfInt) << std::endl;

    // (2) --> 1B
    std::cout << "sizeof(intAliasWrapper) = " << sizeof(intWrapper<int>) << std::endl;
}

In (2), is there a real type compiled or why is this only 1B?

http://cpp.sh/3eiry

Solution

  • The template is a red herring. It's a structure without data members, regardless of T. Since C++ does not allow sizeof(anything)==0, sizeof(intWrapper<int>) has to be greater than 0. This leaves size 1 as a natural choice.