CODEWITHSUNDEEP
c++ooptemplates
forstack overflowizi
forstack overflowizi

Reputation: 424

Template functions' order invoke a compilation error for no reason

so this works fine:

template<class V>
void bar(V v1, V v2) {
}

template <class T>
void foo(T t1, T t2) {
    bar(t1, t2);
}

int main() {
    foo(5, 6);
}

but if i change the order of foo and bar like this:

template <class T>
void foo(T t1, T t2) {
    bar(t1, t2);
}

template<class V>
void bar(V v1, V v2) {
}

int main() {
    foo(5, 6);
}

i get a compilation error:

see reference to function template instantiation 'void foo(T,T)' being compiled

Error C3861 'bar': identifier not found 'bar':

'bar' function was not declared in the template definition context and can be found only via argument-dependent lookup in the instantiation context

what is causing this? what is the hidden problem? what rule of cpp have i broke this time?
I will appreciate insights on the matter

EDIT: thank you for your comments, but for some reason changing bar to swap cause error:
this code:

template<class V>
void swap(V v1, V v2) {
}

template <class T>
void foo(T t1, T t2) {
    swap(t1, t2);
}

int main() {
    foo(5, 6);
}

generates this error:

while trying to match the argument list '(T, T)' see reference to function template instantiation 'void foo(T,T)' being compiled
or 'void std::swapstd::ios_base::iostate,0(_Ty &,_Ty &) noexcept' could be 'void swap(V,V)' Error C2668 'swap': ambiguous call to overloaded function

I know that swap is not a key word in c++.
What is causing this error? i have written this code in a brand new project and it still doesn't work

Upvotes: 2

Views: 1800

Answers (1)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122955

You would get a compiler error from this code for the very same reason:

void foo(T t1, T t2) {
    bar(t1, t2);
}

void bar(V v1, V v2) {
}

int main() {
    foo(5, 6);
}

You cannot call bar before it is declared. Solutions: Either declare bar before you call it, or move the whole definition of bar before foo.

Upvotes: 2

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