What template parameter do I use? (C++ conceptual question)

November 12, 2021

I am going through the book C++ Crash Course by Josh Lospinoso and have been compiling the code in the lessons along the way. I’m having trouble with the following code (which is a simplified version of one of the examples in the book).

struct SmallStruct {};

template <typename T>
struct BigStruct {
    BigStruct(const T& arg) : arg{arg} {};
private:
    const T& arg;
};

int main() {
    BigStruct main_struct{[](const SmallStruct&) {}};
}

The main part of this code I don’t understand is the statement in main(), specifically constructing with a lambda function. I know the code doesn’t compile because when instantiating the BigStruct object in main() it is lacking a template parameter. I’ve tried <SmallStruct>, <SmallStruct&> as parameters but neither compile. If anyone could explain what is going on it would be really beneficial for my learning.

>Solution :

In the "old" days the way was to use a make_... helper function to get the template parameter deduced from a function parameter:

struct SmallStruct {};

template <typename T>
struct BigStruct {
    BigStruct(const T& arg) : arg{arg} {};
private:
    const T& arg;
};

template <typename T>
BigStruct<T> make_big_struct(const T& t){
    return {t};
}

int main() {
   auto main_struct = make_big_struct([](const SmallStruct&) {});
}

Since C++17 there is CTAD (class template argument deduction) and your code compiles without error as is because T can be deduced from the parameter to the constructor (https://godbolt.org/z/oWrnc6bah).