Result of unpacking variadic template - Visual studio 2019 CE

Question

For debugging purposes i want to see the result of unpacking variadic templates in c++. Is it possible anyway in VS?

//variadic template
template<class ... Ts>
void foo(Ts...args)
{
//...some code 
auto x = bar(args..);
}
    
//client code:
foo(std::string("123"),int(4),5.6f);

I supposed result of code generation will be unpacked into something like:

void foo<std::string, int, float>(std::string args1, int args2, float args3)
{
//...some code 
auto x = bar(args1,args2,args3);
}

But if it`s a little harder, who know what it will unpack into? :) Before trying to find this i thought that it will be generated during preprocessing if switching "Properties->C/C++->Preprocessor->Preprocess to a File" to Yes in VS we can see this. But not, on this stage it only work with headers, #incldude''s #define''s ... Question is how to see the result of unpacking parameter packs code generation? Does this code exist in readable form at all ? Or result translated into binary and *.exe file?

Answer 1

What you can do is a kind of static printing.

Static printing that will stop the compilation with an error revealing the params

template <typename...> 
struct TParamViewer;

#define DEBUG_PRINT

#ifdef DEBUG_PRINT
#define VIEW_TPL_PARAMS(index, params) TParamViewer<params> td##index;
#else
#define VIEW_TPL_PARAMS(index, params)
#endif

//variadic template
template<class ... Ts>
void foo(Ts...args)
{
    VIEW_TPL_PARAMS(0, Ts...);
}

Static printing that will show warnings revealing the params

Here we add a definition to the viewer class that will display a warning revealing the template params.

template <typename...> 
struct TParamViewer {
    bool i = 10;
};

All these solutions are just trick. In gcc, there is a proposed patch for proper debug printing.

Regards,