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After searching for an answer for a while, I am now even more confused than before...
I am wondering if I declare in the header file an inline function that is not implemented, but called in the main function, will this result in an linker or compiler error?
When I look at the error I get undefined reference to 'A::a()' it seems like a linker error.
However, in some discussions it says that the Compiler MUST see the function delcaration, therefore it would be an Compiler Error.
I hope you can tell me what I am getting wrong here :)
Header:
Main function:
It is a linker error in most compilers when you have a functon declaration, but the implementation (definition) of the function is not present.
Technically, the compiler MAY report that an inline function is not present in the source, but the compiler is not obliged to do so, and code exists that do not always define the function that is declared as inline. It is not, in most compilers, an error to declare a function inline, and define it in a separate compilation unit (different source file), so compilers do not give an error by default.
If the compiler doesn't find the definition for an inline function, the compiler will assume that it's an external function, and the linker is the only place were all of the code comes together, and thus "it's not there" is issued by the linker.
To clarify:
You may well have a situation where you want to declare as inline a function but put the implementation of a function in a source-file (perhaps because it's never called anywhere else, there is a single factory function to instantiate all instances of that class). Then it is perfectly legal to declare the function [such as a constructor] as inline in the header-file, and the place the implementation of the function in a source file, using the object in various other files without an implementation present.
In practice, compilers also allow the calling of inline functions, even when they are only declared in a source file, not the header. The compiler will not always inline those functions [in practice, only "link time optimization" or "whole program optimization" are likely to do that].
It depends on the compiler you are using but as far as I remember an inline declaration is by default a recommendation to the compiler. In your case, since the compiler didn't find the implementation of the function it will not implement it inline and will leave it to the linker to link.
If you are using GCC you can force the compiler to do inline, see more details in this blog:
Force inline functions in C++(GCC)
I'm currently updating a C++ library for Arduino (Specifically 8-bit AVR processors compiled using avr-gcc).
Typically the authors of the default Arduino libraries like to include an extern variable for the class inside the header, which is defined in the class .cpp file also. This I assume is basically to have everything provided ready to go for newbies as built-in objects.
The scenario I have is: The library I have updated no longer requires the .cpp file and I have removed it from the library. It wasn't until I went on a final pass checking for bugs that I realized, no linker error was produced despite the fact a definition wasn't provided for the extern variable in a .cpp file.
This is as simple as I can get it (header file):
struct Foo{
void method() {}
};
extern Foo foo;
Including this code and using it in one or many source files does not cause any linker error. I have tried it in both versions of GCC which Arduino uses (4.3.7, 4.8.1) and with C++11 enabled/disabled.
In my attempt to cause an error, I found it was only possible when doing something like taking the address of the object or modifying the contents of a dummy variable I added.
After discovering this I find its important to note:
The class functions only return other objects, as in, nothing like operators returning references to itself, or even a copy.
It only modifies external objects (registers which are effectively volatile uint8_t references in code), and returns temporaries of other classes.
All of the class functions in this header are so basic that they cost less than or equal to the cost of a function call, therefore they are (in my tests) completely in-lined into the caller. A typical statement may create many temporary objects in the call chain, however the compiler sees through these and outputs efficient code modifying registers directly, rather than a set of nested function calls.
I also recall reading in n3797 7.1.1 - 8 that extern can be used on incomplete types, however the class is fully defined whereas the declaration is not (this is probably irrelevant).
I'm led to believe that this may be a result of optimizations at play. I have seen the effect that taking the address has on objects which would otherwise be considered constant and compiled without RAM usage. By adding any layer of indirection to an object in which the compiler cannot guarantee state will cause this RAM consuming behavior.
So, maybe I've answered my question by simply asking it, however I'm still making assumptions and it bothers me. After quite some time hobby-coding C++, literally the only thing on my list of do-not's is making assumptions.
Really, what I want to know is:
With respect to the working solution I have, is it a simple case of documenting the inability to take the address (cause indirection) of the class?
Is it just an edge case behavior caused by optimizations eliminating the need for something to be linked?
Or is plain and simple undefined behavior. As in GCC may have a bug and is permitting code that might fail if optimizations were lowered or disabled?
Or one of you may be lucky enough to be in possession of a decoder ring that can find a suitable paragraph in the standard outlining the specifics.
This is my first question here, so let me know if you would like to know certain details, I can also provide GitHub links to the code if needed.
Edit: As the library needs to be compatible with existing code I need to maintain the ability to use the dot syntax, otherwise I'd simply have a class of static functions.
To remove assumptions for now, I see two options:
Add a .cpp just for the variable declaration.
Use a define in the header like #define foo (Foo()) allowing dot syntax via a temporary.
I prefer the method using a define, what does the community think?
Cheers.
Declaring something extern just informs the assembler and the linker that whenever you use that label/symbol, it should refer to entry in the symbol table, instead of a locally allocated symbol.
The role of the linker is to replace symbol table entries with an actual reference to the address space whenever possible.
If you don't use the symbol at all in your C file, it will not show up in the assembly code, and thus will not cause any linker error when your module is linked with others, since there is no undefined reference.
It is either an edge case behaviour caused by optimization, or you never use the foo variable in your code. I'm not 100% sure it is formally not an undefined behavior, but i'm quite sure it isn't undefined from practical point of view.
extern variables are implemented in such way, that code compiled with them produces so-called relocations - empty places where addres of variable should be placed - which are then filled by linker. Apparently foo is never used in your code in such a way that would need getting it's address and therefore linker doesn't even try to find that symbol. If you turn optimization off (-O0) you will probably get linker error.
Update: If you want to keep "dot notation" but remove the problem with undefined extern, you may replace extern with static (in header file), creating separate "instance" of variable for each TU. As this variable is going to be optimized out anyway, this will not change the real code at all, but will also work for unoptimized build.
I was reading the C++ FAQs and I noticed one sentence.
main() cannot be inline.
Why is this?
In C++ it is not legal to call the main function in your code, so there'd be no way it could ever be inlined.
Because the standard says so:
[2003: 3.6.1/3]:
The function main shall not be used (3.2) within a program. The
linkage (3.5) of main is implementation-defined. A program that
declares main to be inline or static is ill-formed. The name main is
not otherwise reserved. [Example: member functions, classes, and
enumerations can be called main, as can entities in other namespaces.
]
And why does it say so? Because it's trying to leave as much about the implementation of main to the individual .. well, implementation .. as is possible, and doesn't want to limit implementations by requiring that inline be valid here when it arguably has no practical benefit.
My friend on the committee confirmed this:
There's no reason why an inline main() wouldn't work, per se. [..] I could have a C++ interpreter that can invoke inlined main(). [..] [But] inline/static main() are forbidden in order to hopefully avoid confusion. I find it hard to imagine that the rationale would be anything additional to what's already been said in [this Q&A].
BTW, don't confuse the inline hint keyword with actually inlining functions. You can mark a function inline and it may not be physically inlined.
So, even if it were true that main "cannot be inlined" (and strictly speaking it is not true, though inlining main would be rather awkward and pointless as explained in other answers), it could theoretically still support the inline hint keyword just fine.
It doesn't for the reason stated above, and in litb's answer: it would complicate matters for no real benefit.
The C runtime library needs to find this symbol in order to "know" which function to run.
You cannot directly call main() (it's forbidden in c++), so there is no point of inlining it.
Usually main() is called from systems init() function. Thus, it is needed that there can be exactly one definition for main().
Now, if we can inline the main() function and include in a header file then, for every translation unit there will be different definition for main(). Which is not allowed. You can declare main() in a namespace and inline it. But not the global main().
firstly you must understand how work function with inline
example:
inline void f() {
int a = 3;
a += 3;
cout << a;
}
int main() {
f();
return 0;
}
will look like to the compiler as:
int main() {
int a = 3;
a += 3;
cout << a;
return 0;
}
looking at this example, how do you want to make main inline? This method is inline immediately.
The C++ standard says that the main function cannot be inlined, per #Tomalak Geret'kal's reply. This response discusses possibility of inlining of the main function, were the restriction in the Standard removed.
Definition of Inline
The inline keyword is a suggestion to the compiler to paste the contents of the function in-situ. One intent is to remove the overhead present in calling and returning from a function (subroutine).
An important situation of inlining is the case where there is a pointer to the function. In this case, there must be at least one static copy of the function. In this case, the linker can resolve "external linkages" of the inlined function because there is one static version.
Important to note that the compiler and linker determine whether or not to paste the contents or calls a single instance of the function.
Also of note, functions that are not tagged by the programmer may also be inlined by the compiler.
Inlining the main function
Since there is only one invocation of main allowed, how it is linked is up to the compiler. Single instances of inline functions are allowed by the Standard. The compiler is allowed to convert an inlined function into a function call to a single instance. So the compiler would ignore an inline suggestion for the main function.
The compiler and linker would have to insure that only one instance of the inlined main function exists. This where the tricky part comes in, especially with external linkage. One process for ensuring one instance is to leave information that a translation has a 'main' function whether or not it is inlined. Note: When a call to an inline function is made, the compiler is allowed to remove the function from the symbol tables for external linkage, since the idea is that the function won't be called by external functions.
Summary
Technically, there is nothing preventing the main function from being inlined. The machinery already exists for converting inlined functions into single instances and for identifying multiple instances of a function. When there is a pointer to an inlined function, a single instance of a function is made, so it has an address. This machinery would satisfy the Run-Time Library requirements for main having an address. In the case of inline for the main function, it would be ignored but there should not be any reason to prevent this syntax (except confusing people). After all, there are already syntax cases that are redundant, such as declaring a parameter that is passed by value (copy) as const.
"That's just my opinion, I could be wrong." -- Dennis Miller, comedian.
Others have remarked that an invocation of main can not meaningfully be inlined at the machine code level. That's rubbish. It would require a bit of help from the linker (like global optimization) or else per-application recompilation of a bit of the runtime library, but it's quite doable, no technical problem here.
However, the hinting effect of inline, that calls should preferably be inlined, is irrelevant for a function that is only called once and at the top level of control, as main is.
The only guaranteed effect of inline is to allow an external linkage function to be defined (identically) in two or more translation units, i.e. affecting the One Definition Rule.
As a practical matter this allows the definition to be placed in a header file, and placing it in a header file is a also practically necessary to guarantee identical definitions.
That does not make sense for main, so there is no reason for main to be inline.
You can only define main once. So putting inline would not serve any purpose - inline only has a significant purpose on functions you can define multiple times in a program (all definitions will be treated as if there were only one definition and all definitions are required to be the same).
Because inline functions can be defined multiple times in a program, and inline also serves the purpose of making calls to an inline-marked function as fast as possible, the Standard requires inline functions to be defined in every translation unit in which it is used. So compilers will usually throw away the definition of a function if it is inline and the function wasn't used by the code in the current translation unit. To do that for main would be entirely wrong, which goes to show that inline and the semantics main has is entirely incompatible.
Note that the question in your title "Why main() in C++ cannot be inlined?" and the statement you quote out of the Standard concern different things. You are asking whether the function can be inlined, which commonly is understood to insert the code of a called function completely or partially into the calling function. Just marking a function inline doesn't imply inlining that function at all. It's entirely the compiler's decision, and of course if you never call main (and you cannot do so) then there is nothing to be inlined.
If you linked statically to the CRT and enabled some link-time compilation-inlining (like MSVC has) it might be possible to inline it.
But it doesn't really make sense. It will be called once and that function call-overhead is practically naught compared to everything else that is done before the first line in main executes.
...
Aaand, it is an easy way to force the symbol to appear only once in your executable. :)
There are a number of basic reasons. Basically, main is called from
the basic initialization routine of the runtime, and only from there.
That code was (obviously) compiled without knowing that your main was
inlined. Modern compiler technology is capable of inlining across
module boundaries, but it's an advanced feature, not supported by many
older compilers. And of course, the benefits of inlining are only
present when a function is called very frequently; by definition, main
will be called exactly once, no more, no less.
I see the standard says so, but the real practical answer would be as simple as stating that the runtime added to every C and C++ program has to call to some point in the executable. That function should have an external symbol (and address when running) so that the linker can find it to be called at the beginning of execution. Hence you cannot declare it as inline, because inlined the compiler wouldn't generate an external symbol for it.
Since its the main() function, which starts the execution, when the code gets compiled to binary, everything is in the main() itself. so you can say, it already inlined!
And yes, its illegal to use inline for your C++ program, that's more about a syntax!
For most combinations of compiler/archetecture, the main() function in the source becomes a reasonably normal function in the final binary. This is only because it's convenient on those archetectures, not because the standard says it must be so.
On memory constrained archetectures, many compilers, ones which produce a flat binary (like intex hex format) instead of a dynamic linker friendly container (like elf or xcoff), optimize all of the boilerplate away, since it would just be bloat. Some architectures don't support function calls at all (only a limited subset of C++ is possible on these platforms.)
In order to support the widest variety of such architectures and build environments, the standard elects keep the semantics of main() as open as possible, so that the compiler can do what's right for the widest variety of platforms. That means that many features available in the language as a whole cannot apply to the startup and shutdown of the application itself.
If you need something like an inline main() (or reentrancy, or any fancy feature) you can of course call the main function something else:
inline int myMain(int argc, char **argv) { /* whatever */ }
int main(int argc, char **argv) { return myMain(argc, argv); }
Inline functions are having static scope by-default. It means if we declare main() as inline, it's scope will be limited to the file where it is defined. Yet, the C start-up library (provided by compiler vendor) needs 'main' to be a global symbol. There are some compilers that allow to modify entry point function (e.g. main) using linker flags.
inline functions don't usually have an address, so there is no portable way to call main, main() needs an address on which the init code can jump into. Inlined functions are meant to be stuck into the calling function, if main is inlined, it should be inlined into the init code of the program, which is not portable either.
operating system loads binary data to memory; looks for entry point (the 'main' symbol in c/c++); makes far jump to the addres of the entry point label. Operating system does not know anything about main function in your code until the program is not loaded.
When reading through some answers to this question, I started wondering why the compiler actually does need to know about a function when it first encounters it. Wouldn't it be simple to just add an extra pass when parsing a compilation unit that collects all symbols declared within, so that the order in which they are declared and used does not matter anymore?
One could argue, that declaring functions before they are used certainly is good style, but I am wondering, is there are any other reason why this is mandatory in C++?
Edit - An example to illustrate: Suppose you have to functions that are defined inline in a header file. These two function call each other (maybe a recursive tree traversal, where odd and even layers of the tree are handled differently). The only way to resolve this would be to make a forward declaration of one of the functions before the other.
A more common example (though with classes, not functions) is the case of classes with private constructors and factories. The factory needs to know the class in order to create instances of it, and the class needs to know the factory for the friend declaration.
If this is requirement is from the olden days, why was it not removed at some point? It would not break existing code, would it?
How do you propose to resolve undeclared identifiers that are defined in a different translation unit?
C++ has no module concept, but has separate translation as an inheritance from C. A C++ compiler will compile each translation unit by itself, not knowing anything about other translation units at all. (Except that export broke this, which is probably why it, sadly, never took off.)
Header files, which is where you usually put declarations of identifiers which are defined in other translation units, actually are just a very clumsy way of slipping the same declarations into different translation units. They will not make the compiler aware of there being other translation units with identifiers being defined in them.
Edit re your additional examples:
With all the textual inclusion instead of a proper module concept, compilation already takes agonizingly long for C++, so requiring another compilation pass (where compilation already is split into several passes, not all of which can be optimized and merged, IIRC) would worsen an already bad problem. And changing this would probably alter overload resolution in some scenarios and thus break existing code.
Note that C++ does require an additional pass for parsing class definitions, since member functions defined inline in the class definition are parsed as if they were defined right behind the class definition. However, this was decided when C with Classes was thought up, so there was no existing code base to break.
Historically C89 let you do this. The first time the compiler saw a use of a function and it didn't have a predefined prototype, it "created" a prototype that matched the use of the function.
When C++ decided to add strict typechecking to the compiler, it was decided that prototypes were now required. Also, C++ inherited the single-pass compilation from C, so it couldn't add a second pass to resolved all symbols.
Because C and C++ are old languages. Early compilers didn't have a lot of memory, so these languages were designed so a compiler can just read the file from top to bottom, without having to consider the file as a whole.
I think of two reasons:
It makes the parsing easy. No extra pass needed.
It also defines scope; symbols/names are available only after its declaration. Means, if I declare a global variable int g_count;, the later code after this line can use it, but not the code before the line! Same argument for global functions.
As an example, consider this code:
void g(double)
{
cout << "void g(double)" << endl;
}
void f()
{
g(int());//this calls g(double) - because that is what is visible here
}
void g(int)
{
cout << "void g(int)" << endl;
}
int main()
{
f();
g(int());//calls g(int) - because that is what is the best match!
}
Output:
void g(double)
void g(int)
See the output at ideone : http://www.ideone.com/EsK4A
The main reason will be to make the compilation process as efficient as possible. If you add an extra pass you're adding both time and storage. Remember that C++ was developed back before the time of Quad Core Processors :)
The C programming language was designed so that the compiler could be implemented as a one-pass compiler. In such a compiler, each compilation phase is only executed once. In such a compiler you cannot referrer to an entity that is defined later in the source file.
Moreover, in C, the compiler only interpret a single compilation unit (generally a .c file and all the included .h files) at a time. So you needed a mechanism to referrer to a function defined in another compilation unit.
The decision to allow one-pass compiler and to be able to split a project in small compilation unit was taken because at the time the memory and the processing power available was really tight. And allowing forward-declaration could easily solve the issue with a single feature.
The C++ language was derived from C and inherited the feature from it (as it wanted to be as compatible with C as possible to ease the transition).
I guess because C is quite old and at the time C was designed efficient compilation was a problem because CPUs were much slower.
Since C++ is a static language, the compiler needs to check if values' type is compatible with the type expected in the function's parameters. Of course, if you don't know the function signature, you can't do this kind of checks, thus defying the purpose of a static compiler. But, since you have a silver badge in C++, I think you already know this.
The C++ language specs were made right because the designer didn't want to force a multi-pass compiler, when hardware was not as fast as the one available today. In the end, I think that, if C++ was designed today, this imposition would go away but then, we would have another language :-).
One of the biggest reasons why this was made mandatory even in C99 (compared to C89, where you could have implicitly-declared functions) is that implicit declarations are very error-prone. Consider the following code:
First file:
#include <stdio.h>
void doSomething(double x, double y)
{
printf("%g %g\n",x,y);
}
Second file:
int main()
{
doSomething(12345,67890);
return 0;
}
This program is a syntactically valid* C89 program. You can compile it with GCC using this command (assuming the source files are named test.c and test0.c):
gcc -std=c89 -pedantic-errors test.c test0.c -o test
Why does it print something strange (at least on linux-x86 and linux-amd64)? Can you spot the problem in the code at a glance? Now try replacing c89 with c99 in the command line — and you'll be immediately notified about your mistake by the compiler.
Same with C++. But in C++ there're actually other important reasons why function declarations are needed, they are discussed in other answers.
* But has undefined behavior
Still, you can have a use of a function before it is declared sometimes (to be strict in the wording: "before" is about the order in which the program source is read) -- inside a class!:
class A {
public:
static void foo(void) {
bar();
}
private:
static void bar(void) {
return;
}
};
int main() {
A::foo();
return 0;
}
(Changing the class to a namespace doesn't work, per my tests.)
That's probably because the compiler actually puts the member-function definitions from inside the class right after the class declaration, as someone has pointed it out here in the answers.
The same approach could be applied to the whole source file: first, drop everything but declaration, then handle everything postponed. (Either a two-pass compiler, or large enough memory to hold the postponed source code.)
Haha! So, they thought a whole source file would be too large to hold in the memory, but a single class with function definitions wouldn't: they can allow for a whole class to sit in the memory and wait until the declaration is filtered out (or do a 2nd pass for the source code of classes)!
I remember with Unix and Linux, you have Global and Local. Within your own environment local works for functions, but does not work for Global(system). You must declare the function Global.
I'm getting linkage errors of the following type:
Festival.obj : error LNK2019:
unresolved external symbol "public:
void __thiscall Tree::add(class Price &)"
(?add#?$Tree#VPrice####QAEXAAVPrice###Z)
referenced in function
__catch$?AddBand#Festival##QAE?AW4StatusType##HHH#Z$0
I used to think it has to do with try-catch mechanism, but since been told otherwise. This is an updated version of the question.
I'm using Visual Studio 2008, but I have similar problems in g++.
The relevant code:
In Festival.cpp
#include "Tree.h"
#include <exception>
using namespace std;
class Band{
public:
Band(int bandID, int price, int votes=0): bandID(bandID), price(price), votes(votes){};
...
private:
...
};
class Festival{
public:
Festival(int budget): budget(budget), minPrice(0), maxNeededBudget(0), priceOffset(0), bandCounter(0){};
~Festival();
StatusType AddBand(int bandID, int price, int votes=0);
...
private:
Tree<Band> bandTree;
...
};
StatusType Festival::AddBand(int bandID, int price, int votes){
if ((price<0)||(bandID<0)){
return INVALID_INPUT;
}
Band* newBand=NULL;
try{
newBand=new Band(bandID,price-priceOffset,votes);
}
catch(bad_alloc&){return ALLOCATION_ERROR;}
if (bandTree.find(*newBand)!=NULL){
delete newBand;
return FAILURE;
}
bandTree.add(*newBand);
....
}
In Tree.h:
template<class T>
class Tree{
public:
Tree(T* initialData=NULL, Tree<T>* initialFather=NULL);
void add(T& newData);
....
private:
....
};
Interestingly enough I do not have linkage errors when I try to use Tree functions when type T is a primitive type like an int.
Is there Tree.cpp? If there is, maybe you forgot to link it? Where is the implementation of Tree::add?
In addition I don't see where you call Tree::add. I guess it should be inside the try statement, right after the new?
Just a reminder:
For most compilers (i.e. those that practice separate compilation) the implementation of the member functions of a template class has to be visible during the compilation of the source file that uses the template class. Usually people follow this rule by putting the implementation of the member functions inside the header file.
Maybe Tree::add isn't inside the header? Then a possible solution in the discussed case will be to put Tree::add implementation inside the header file.
The difference between regular classes and template classes exists because template classes are not "real" classes - it is, well, a template. If you had defined your Tree class as a regular class, the compiler could have used your code right away. In case of a template the compiler first "writes" for you the real class, substituting the template parameters with the types you supplied. Now, compiler compiles cpp files one by one. He is not aware of other cpp files and can use nothing from other cpp files. Let's say your implementation of Tree:add looks like this:
void Tree::add(T& newData)
{
newData.destroyEverything();
}
It is totally legitimate as long as your T has method destroyEverything. When the compiler compiles Class.cpp it wants to be sure that you don't do with T anything it doesn't know. For example Tree<int> won't work because int doesn't have destroyEverything. The compiler will try to write your code with int instead of T and find out that the code doesn't compile. But since the compiler "sees" only the current cpp and everything it includes, it won't be able to validate add function, since it is in a separate cpp.
There won't be any problem with
void Tree::add(int& newData)
{
newData.destroyEverything();
}
implemented in a separate cpp because the compiler knows that int is the only acceptable type and can "count on himself" that when he gets to compile Tree.cpp he will find the error.
Are you sure the try/catch has anything to do with it? What happens if you simply comment out the try and catch lines, leave the rest of the code as it is, and build that?
It might just be that you're missing the library that defines Tree::add(class Price &) from your link line.
Update: using Tree functions with a primitive type doesn't result in a linking error.
I updated my question in light of some of the things that were said.
As others have stated you need to show the implementation of Treee::add() and tell us how you are linking it.
On an unrelated point, if you are using constructs like:
Band* newBand=NULL;
try{
newBand=new Band(bandID,price-priceOffset,votes);
}
catch(bad_alloc&){return ALLOCATION_ERROR;}
throughout your code, you are frankly wasting your time. The chances of you getting to a point of memory exhaustion in a modern OS are remote and the chances of you doing anything useful after it has happened are roughly zero. You will be much better off simply saying:
Band * newBand = new Band ( bandID, price - priceOffset, votes );
ot possibly:
Band newBand( bandID, price - priceOffset, votes );
and forgetting the exception handling in this case.
You wrote in a comment:
I considered this but the function is part of Tree.h, and I do include it. The function defined is: template void Tree::add(T& newData); We call it the following way: priceTree.add(*newPriceNode); whereas priceTree is Tree, both of which are defined in the cpp file in question.
instead of:
priceTree.add(*newPriceNode);
try:
priceTree.add(newPriceNode); //no "*" before "newPriceNode"
add() takes a reference to a node, not a pointer to a node (according to your definition of Tree).
You're getting linkage errors, not compiler errors. This tells us that the compiler knew what sort of function Tree::add() is, but didn't have a definition. In Tree.h, I see a declaration of the add() function, but not a definition. It looks odd to me; does anybody know where Tree.h came from?
Usually a template class comes with member function definitions in the include file, since the functions have to be instantiated somewhere, and the simplest thing is for the compiler to instantiate when used and let the linker sort it out. If the definitions are in Tree.h, I'd expect everything to work as planned.
So, I'm going to go out on a limb and suggest that the definitions are in a separate file, not linked in, and that there are provisions elsewhere for instantiating for basic types like Tree<int>. This is presumably to streamline compilation, as normally these things are compiled in multiple places, and that takes time.
What you need to do in that case is to find where Tree<int> is instantiated, and add an instantiation for your class.
I could be way off base here, but my explanation does fit the facts you've given.
Edit after first comments:
Templates are somewhat trickier than ordinary functions, which usually isn't a real problem. If the definitions for all the calls were in Tree.h, then Festival.cpp would be able to instantiate Tree<Band> and everything would be cool. That's the usual technique, and you're running into this problem because you're not using it.
When you write a function, it gets compiled, and the linker will find it. Any routine calling that function needs to know the function prototype, so it will know how to call it. When you write a template, you're not writing anything that will go directly into the program, but any use of the template counts as writing all the functions.
Therefore, there has to be some use of Tree<Band> somewhere in your program, for there to be a Tree<Band>::add() function compiled. The definition of Tree<T>::add has to be available to the compiler when Tree<Band> is instantiated, because otherwise the compiler has no idea what to compile. In this case, it's generating the function call, confident that you'll make sure the function is compiled elsewhere.
Therefore, you have to instantiate Tree<Band> inside a file that has access to both the definitions for Tree<T> and Band. This probably means a file that is, or includes, Tree.cpp and includes Festival.h.
The linker is already using Tree.cpp, but Tree.cpp doesn't have Tree<Band> defined in it, so it's meaningless to the linker. Templates are only useful for the compiler, and the linker only operates on what the compiler generated from templates.
The quick way to solve this is to take the definitions from Tree.cpp and put them in Tree.h. That will be likely to increase compilation and link times, unfortunately. The other technique is to instantiate all template uses in Tree.cpp, so that they'll be compiled there.