Template instantiation

[broken ridge]

I have doubt about this snippet taken from C++ Templates: The complete Guide.
How come foo parameter type is not void**. I find it confusing since I'm assuming when we call foo here
and pass vp as parameter, isn't void* the actual type of vp, so it should deduce void foo(void**) instead?

[spice patrol]

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[undone shale]

well... when you call the function

foo(vp);

this does not always mean that the type T will be substituted with decltype(vp) which here is void*, C++ is trying to find the best match for the template, and here the best match is T = void

if you really want to be explicit and ban any form of template deduction or guessing, you can do this instead:

foo<void*>(vp);

and now the type T will be guaranteed to be void* but then T* will be void** and your vp parameter cannot be inplicitly converted to void** so you will get a compilation error, you gotta use &vp instead

[broken ridge]

ahh

i understandit better now, ty

[undone shale]

the automatic template deduction is a feature of C++11, C++98 did not have that

so in C++98 you were forced to do shit like

int arr[] = {2,5,4,7,1};
std::sort<int*>(arr, arr+5);

[broken ridge]

there should be a long set rules for arguments match im guessing, my brain just directly substituted types

[undone shale]

yes, template deduction guidelines in cppreference

very long, very boring

learn templates by writing code instead pls

[broken ridge]

agree

i am

was consulting the book out of curiosity

[undone shale]

oh and

[lyric meadow]

i think the main thing you need to keep in mind is the above

[undone shale]

have you ever asked yourself "that template T, what is T exactly?"

@broken ridge

[broken ridge]

no, honestly

[undone shale]

well...

as you begin learning more about templates you will probably hit this question one day

the approach I use is

write a compilation error in the function

compile

and in the compilation error it show all of the templates substituted 🙂

[spice patrol]

@broken ridge Has your question been resolved? If so, run !solved :)

[broken ridge]

!solved