[SOLVEDJ Arduino IDE: C++ template bug or quirks??

Projects Programming
1

Why doesn't this code compile? I tried pretty much the same code in Visual Studio VS2017 and it compiles fine. What's going on??

Tried with Arduino 1.8.5 Windows.

template <typename T>
struct Foo
{
  struct Bar
  {
    T data_;
  };
  Bar bar_;
};

template <typename T>
void func(typename Foo<T>::Bar* bar)
{
}

void setup()
{
}

void loop() {
  Foo<int> foo;
  foo.bar_.data_ = 17;
  func<int>(&foo.bar_);
}

I get the following errors:

circular_buffer_with_ids_test:12: error: 'T' was not declared in this scope

 void func(typename Foo<T>::Bar* bar)

                        ^

circular_buffer_with_ids_test:12: error: template argument 1 is invalid

 void func(typename Foo<T>::Bar* bar)

                         ^

exit status 1
'T' was not declared in this scope
2

I have no problem compiling it in 1.8.5....

Maybe post a MCVE and the actual error.

3

Hi septillion,

So if you copy and past that code i posted above into a sketch, you can compile?

strange.

ps i posted the errors above

4

Typically, all templated function definitions and implementations are put in a header file which is included in the sketch.

Then, the IDE CAN compile your code.

5

Interesting. I just copied and pasted into 1.8.4, 1.8.5 and 1.8.6.

Errors in 1.8.4 and 1.8.5. None with 1.8.6 (unused parameter warning).

6

Previous versions of the Arduino IDE had problems with multi-line template signatures in .ino files. The workaround was to put the entire signature on one line:

template <typename T> struct Foo
{
  struct Bar
  {
    T data_;
  };
  Bar bar_;
};

template <typename T> void func(typename Foo<T>::Bar* bar)
{
}

void setup()
{
}

void loop() {
  Foo<int> foo;
  foo.bar_.data_ = 17;
  func<int>(&foo.bar_);
}

Compiles fine in Arduino IDE 1.8.5.

Templates will always work as expected in .h files because the Arduino IDE only does sketch preprocessing on .ino files.

7

I have found in the past the compiler is a bit weird when the template declaration i split across two lines...

template <typename T> void func(typename Foo<T>::Bar* bar)
{
}

Is fine

template <typename T> 
void func(typename Foo<T>::Bar* bar)
{
}

Is not..

[edit] pert was first :smiley:

8

Thanks gang

hint from PaulS to put in header file solved.

Conclusion: Arduino IDE quirk?!

This below worked, once put in a_header_file.h and placed in libraries folder:

template <typename T>
struct Foo
{
  struct Bar
  {
    T data_;
  };
  Bar bar_;
};

template <typename T>
void func(typename Foo<T>::Bar* bar)
{
}
9

Just to confirm,

template void func(typename Foo::Bar* bar)
{
}

now compiles 1.8.4, 1.8.5, 1.8.6