Multiple definition exception in C++ - c++

I am starting to learn C++ and am getting this compilation error from my IDE (CodeBlocks). I don't understand why this is happening.
|2|multiple definition of `parser::parseFile()'
|2|first defined here|
I don't see how this could happen. This is my entire code base.
main.cpp
#include "parser/parser.cpp"
int main()
{
return 0;
}
parser/parser.cpp
namespace parser {
void parseFile() {
}
}

Assuming you compiled both main.cpp and parser/parse.cpp you clearly have two definitions of parser::parseFile(): the #include directive just become replaced by the content of the named file (you can use the -E flag with your compiler to see the result).
You probably meant to declare parser::parseFile() in a header file (typically with a suffix .h or .hpp or something like that):
// file: parser/parser.hpp
#ifndef INCLUDED_PARSER_PARSER
#define INCLUDED_PARSER_PARSER
namespace parser {
void parseFile();
}
#endif
... and to include this header file into both translation units.

Your program have violated the One Definition Rule (also known as ODR).
In short, parser::parseFile function have been defined in both of your .cpp files, because on the compiler level, #include <header.h> simply means substituting the entire file contents in place.
Solution to your problem depends on your actual program, though. If you want to solve the ODR rule for class definitions, you can do either of:
1) Add a #pragma once at the beginning on a header. This, although being supported by all major compilers, is not standardized way of protecting from double-including a header.
2) Add an include guard:
#ifndef MY_HEADER_GUARD
#define MY_HEADER_GUARD
// your header contents go here
#endif
If you want to solve the ODR problem for functions and data variables, the above approach won't work because you can still have them defined multiple times in different .cpp files.
For this, you still have 2 options:
1) define your function somewhere outside, namely, in some .cpp file, only leaving its declaration in the header:
// header.h
namespace parser {
void func();
}
// file.cpp
void parser::func() { ... }
2) Declare your function as inline, as inline function are allowed to have multiple definitions by the C++ standard (however, they must be strictly identical up to lexem level):
// header.h
namespace parser {
inline void func() { ... }
}
To sum it up, I'd strongly recommend you go both directions by protecting your header from double inclusion and making sure your function is either inline or gets defined in a .cpp file. With the latter, if your function implementation changes, you won't have to recompile all the files that include your header, but only the one that has the functon definition.

Header files ending with .h(usually) is often used to separate class and function declaration from their implementation.
In Code::Blocks you can add header files by right clicking on your project name -> 'Add files' -> create a new file with .h extension. Following good practise, you should also create a .cpp file with the same name where the implementation is written.
As already answered, in your header file you can first write an include guard followed by your include's (if any) and also your function declarations. And in your 'parser.cpp' & 'main.cpp' you will have to #include "parser.h" since the files depend on eachother.

Related

Use header-only (+implementation .h files) across multiple files in C++ project [duplicate]

I have a c++ header file containing a class.
I want to use this class in several projects, bu I don't want to create a separate library for it, so I'm putting both methods declarations and definitions in the header file:
// example.h
#ifndef EXAMPLE_H_
#define EXAMPLE_H_
namespace test_ns{
class TestClass{
public:
void testMethod();
};
void TestClass::testMethod(){
// some code here...
}
} // end namespace test_ns
#endif
If inside the same project I include this header from more than one cpp file, I get an error saying "multiple definition of test_ns::TestClass::testMethod()", while if I put the method definition inside the class body this does not happen:
// example.h
#ifndef EXAMPLE_H_
#define EXAMPLE_H_
namespace test_ns{
class TestClass{
public:
void testMethod(){
// some code here...
}
};
} // end namespace test_ns
#endif
Since the class is defined inside a namespace, shouldn't the two forms be equivalent? Why is the method considered to be defined twice in the first case?
Inside the class body is considered to be inline by the compiler.
If you implement outside of body, but still in header, you have to mark the method as 'inline' explicitly.
namespace test_ns{
class TestClass{
public:
inline void testMethod();
};
void TestClass::testMethod(){
// some code here...
}
} // end namespace test_ns
Edit
For myself it often helps to solve these kinds of compile problems by realizing that the compiler does not see anything like a header file. Header files are preprocessed and the compiler just sees one huge file containing every line from every (recursively) included file. Normally the starting point for these recursive includes is a cpp source file that is being compiled.
In our company, even a modest looking cpp file can be presented to the compiler as a 300000 line monster.
So when a method, that is not declared inline, is implemented in a header file, the compiler could end up seeing void TestClass::testMethod() {...} dozens of times in the preprocessed file. Now you can see that this does not make sense, same effect as you'd get when copy/pasting it multiple times in one source file.
And even if you succeeded by only having it once in every compilation unit, by some form of conditional compilation ( e.g. using inclusion brackets ) the linker would still find this method's symbol to be in multiple compiled units ( object files ).
These are not equivalent. The second example given has an implicit 'inline' modifier on the method and so the compiler will reconcile multiple definitions itself (most likely with internal linkage of the method if it isn't inlineable).
The first example isn't inline and so if this header is included in multiple translation units then you will have multiple definitions and linker errors.
Also, headers should really always be guarded to prevent multiple definition errors in the same translation unit. That should convert your header to:
#ifndef EXAMPLE_H
#define EXAMPLE_H
//define your class here
#endif
Don't put a function/method definition in an header file unless they are inlined (by defining them directly in a class declaration or explicity specified by the inline keyword)
header files are (mostly) for declaration (whatever you need to declare). Definitions allowed are the ones for constants and inlined functions/methods (and templates too).
Actually it is possible to have definitions in a single header file (without a separate .c/.cpp file) and still be able to use it from multiple source files.
Consider this foobar.h header:
#ifndef FOOBAR_H
#define FOOBAR_H
/* write declarations normally */
void foo();
void bar();
/* use conditional compilation to disable definitions when necessary */
#ifndef ONLY_DECLARATIONS
void foo() {
/* your code goes here */
}
void bar() {
/* your code goes here */
}
#endif /* ONLY_DECLARATIONS */
#endif /* FOOBAR_H */
If you use this header in only one source file, include and use it normally.
Like in main.c:
#include "foobar.h"
int main(int argc, char *argv[]) {
foo();
}
If there're other source files in your project which require foobar.h, then #define ONLY_DECLARATIONS macro before including it.
In use_bar.c you may write:
#define ONLY_DECLARATIONS
#include "foobar.h"
void use_bar() {
bar();
}
After compilation use_bar.o and main.o can be linked together without errors, because only one of them (main.o) will have implementation of foo() and bar().
That's slightly non-idiomatic, but it allows to keep definitions and declarations together in one file. I feel like it's a poor man's substitute for true modules.
Your first code snippet is falling foul of C++'s "One Definition Rule" - see here for a link to a Wikipedia article describing ODR. You're actually falling foul of point #2 because every time the compiler includes the header file into a source file, you run into the risk of the compiler generating a globally visible definition of test_ns::TestClass::testMethod(). And of course by the time you get to link the code, the linker will have kittens because it will find the same symbol in multiple object files.
The second snippet works because you've inlined the definition of the function, which means that even if the compiler doesn't generate any inline code for the function (say, you've got inlining turned off or the compiler decides the function is too big to inline), the code generated for the function definition will be visible in the translation unit only, as if you'd stuck it in an anonymous namespace. Hence you get multiple copies of the function in the generated object code that the linker may or may not optimize away depending on how smart it is.
You could achieve a similar effect in your first code snippet by prefixing TestClass::testMethod() with inline.
//Baseclass.h or .cpp
#ifndef CDerivedclass
#include "Derivedclass.h"
#endif
or
//COthercls.h or .cpp
#ifndef CCommonheadercls
#include "Commonheadercls.h"
#endif
I think this suffice all instances.

Why static member can't be defined in headerfile with #pragma once

Staitc member has to defined in a sourcefile(.cpp) because it will cause error due to duplicate definitions since headerfile(.h) is shared with many sourcefiles. However, if i type '#pragma once
at the beginning of the heaerfile (or use #ifndef, #endif). it will only run the headerfile once. Then why we still need to initialize static member in a sourcefile?
// This is string1.h
#pragma once
#include <cstring> // string.h for some
using std::ostream;
using std::istream;
static int num_strings=1; //why this still gives multiple definiton error even with #proagma once at top?
class String
{
private:
static int num_strings; // number of objects
}
#pragma once does not stop a header file from being compiled multiple times.
If you have two .cpp files and each includes a header file, then that header file will be compiled twice. It cannot be any other way because C++ uses separate compilation for each .cpp file. So what happens in one compilation cannot affect what happens in another compilation.
What #pragma once does is stop a header file being compiled twice during a single compilation. So your won't get errors if you include a header twice in the same .cpp file. This happens more often than you might think. For instance a .cpp file could include two different header files, but each of those header files includes a third common header file. So indirectly the common header file is being included twice in the .cpp file. It's this problem that #pragma once is designed to fix.
Your line of reasoning is wrong. Header guards are to prevent including the header more than once in a single translation unit. Suppose you have two source files:
// foo.cpp
#include "theheader.h"
// bar.cpp
#include "theheader.h"
#include "someotherheader.h"
Then those two will be compiled seperately, each has the contents of the header. The header guard only prevents to include "theheader.h" twice if for example the other header is
// someotherheader.h
#include "theheader.h"
To summarize, I allow myself to steal from a comment: "Include guards avoid that file is included multiple time in same translation unit, not across several translation unit"
Since C++17 you can get around the ODR violation by making the variable inline:
static inline int num_strings=1;
class String
{
private:
static inline int num_strings{}; // number of objects
}
See this for more information.
Btw, those are two distinct variables. One is num_strings and the other is String::num_strings.

gcc Multiple definition...release only

I have defined a function within a header file. In debug, it compiles and links fine. In release, I get the compile error "multiple definition of `blah::blah(blah& blah)'" for every object file of every class that includes the header.
I'm using gcc-4.8.1. I can't post up the actual code, this version has names changed to protect the innocent:
#ifndef INCLUDE_SMELLS_FUNNY
#define INCLUDE_SMELLS_FUNNY
#include "ParentClass.h"
#include "ChildClassA.h"
#include "ChildClassB.h"
namespace someplace {
bool smellsFunny(const ParentClass& someData) {
// All ChildClass As smell funny
if(dynamic_cast<const ChildClassA*>(&someData)) {
return true;
}
// If ChildClass B, need to check if smells funny
const ChildClassB* childB = dynamic_cast<const ChildClassB*>(&someData)
if(childB) {
return childB->MoreThanAWeekOld();
}
// Default is smells OK
return false;
}
}
#endif // INCLUDE_SMELLS_FUNNY
I haven't been able to find which gcc flag is responsible.
Of course, a fix is just to move the implementation into a cpp file. But why is this necessary? Why does this happen only in release?
It's because you define the function in a header file, and then include that header file in multiple source files. That means the function will be defined in each source file (technically, translation unit) where you include the header.
There are a couple of solutions to this:
Mark the function as static. That means that each definition in the translation units will not be exported.
Mark the function as inline. This works basically the same as the first alternative.
Put the definition of the function in a source file, and only have its declaration in the header file.
For a small function like the one you have, then alternative 2 might be a good one. If you have a larger function, that can't easily be inlined, you should go with alternative 3.
Also, if you go with number 3, then you don't need to include the header files, and then lessen the risk of circular inclusion.
Since you have placed the function definition in the header, you need to mark it inline:
inline bool smellsFunny(const ParentClass& someData) { ...
This is because every place in which you include the header will have a definition of the function, breaking the one definition rule (ODR). inline lets you work around the ODR.
As to why there is no problem in release mode, that doesn't make sense: the code is not correct. It could be that release mode is declaring the function inline for you.
I can not say exactly but maybe in the debug mode the function is considered as inline function.
If the function is in a header file, you need to declare it inline, unless it's defined within a class.

Multiple Source .cpp Files Problems

I've separated my large program into a number of .cpp files, e.g. display.cpp, gui.cpp, display.h, gui.h, etc...
These files are separated just for readability, not necessarily indicative of any sort of dependency scheme.
Initially I had a lot of trouble getting them to compile, because functions from display will call functions from gui, AND vice versa. Whichever one I include first, it will still depend on the other's functions. But I finally figured out that I need to first include all the .h files, then include all the .cpp files, in my main program.
Example, in the main_program.cpp:
#include "display.h"
#include "gui.h"
#include "display.cpp"
#include "gui.cpp"
Unfortunately I also realized that in order to compile I had to remove all the other .cpp files from what is considered "source" code in the Visual Studio debugger. That way it just compiles main_program.cpp, including the other files as needed. If I include display.cpp and gui.cpp in the "source" sidebar it will error.
This is probably the wrong way of doing things and I feel like I am doing something wrong. I would like to be able to put all the source files in the sidebar and still have it compile. Of course the above was just an example and I have not two, but more like 10 different .cpp files that all call each others' functions. Please advise on how to better design this.
You generally never want to include .cpp files! Also, you should get used to factoring your code according to dependency hierarchies which shall not include any cycle! For small proframs this is just helpful, for large software it is essential.
The problem is that you've included .cpp files, that's why you had to tell Visual Studio to pretend that these files were not "source" code, because it always compiles source code files. So the simple solution is don't do that.
Including just the header files (.h) in each .cpp file as required should be sufficient. So, for example, if gui.cpp needs to call functions defined in display.cpp, then you should #include the display.h header file at the top of the gui.cpp code file.
Use forward declarations to eliminate any lingering circular dependencies.
That is... not the right way to approach things. If I had to speculate, I'd guess you aren't using function declarations that aren't definitions. A function definition is what you probably have:
void dothing(int param) { //function definition
throw param;
}
What goes in the header is the function declaration, which would be like this:
void dothing(int param); //function declaration
This merely lets other cpp files know that the file exists for calling, but they don't need to know the details. Generally, functions will go in your .cpp files as you (seem to have) done. Each function will have a declaration in a header file:
display.h
#include "velocity.h" //we need the details, so we include
void dothing(int param); //function declaration
class coordinate; // this is a class declaration
//this header needs to know about coordinates
//but doesn't need the details
//otherwise we'd have to include "coordinate.h"
struct object { //class definitions usually go in headers
coordinate* member; //pointers don't need to know the details
velocity speed; //actual object needs the details
float function(float param); //function declaration
};
display.cpp
void dothing(int param) { //function definition
throw param;
}
float object::function(float param) { //member function definition
}
Then, if a cpp file neads access to a function in another cpp file, it includes the appropriate header:
main.cpp
#include "display.h"
int main() {
object a;
a.function(3.4); //can call the member just fine
dothing(4); //it can call the function just fine.
}
Remember that headers should prefer declaring a class (in my example: coordinate to including more headers wherever possible. Also remember that templates have completely different rules altogether.
I had a similar problem before in which I had two header files referencing each other, calling functions, etc. To fix it, I first made sure that all of my header guards were in check (#ifndef HEADER_DEFINE, #pragma once, etc.), then in the class files I would include the header that was being problematic.
e.g. if I have Pie.h and Cherries.h, and Pie.h included Cherries.h and vice versa, then in the Pie.cpp file, I had the include for Cherries.h (and vice versa).

How can I avoid including class implementation files?

Instead of doing
#include "MyClass.cpp"
I would like to do
#include "MyClass.h"
I've read online that not doing so is considered bad practice.
Separate compilation in a nutshell
First, let's get some quick examples out there:
struct ClassDeclaration; // 'class' / 'struct' mean almost the same thing here
struct ClassDefinition {}; // the only difference is default accessibility
// of bases and members
void function_declaration();
void function_definition() {}
extern int global_object_declaration;
int global_object_definition;
template<class T> // cannot replace this 'class' with 'struct'
struct ClassTemplateDeclaration;
template<class T>
struct ClassTemplateDefinition {};
template<class T>
void function_template_declaration();
template<class T>
void function_template_definition() {}
Translation Unit
A translation unit (TU) is a single source file (should be a **.cpp* file) and all the files it includes, and they include, etc. In other words: the result of preprocessing a single file.
Headers
Include guards are a hack to work around lack of a real module system, making headers into a kind of limited module; to this end, including the same header more than once must not have an adverse affect.
Include guards work by making subsequent #includes no-ops, with the definitions available from the first include. Because of their limited nature, macros which control header options should be consistent throughout a project (oddball headers like <assert.h> cause problems) and all #includes of public headers should be outside of any namespace, class, etc., usually at the top of any file.
See my include guard naming advice, including a short program to generate include guards.
Declarations
Classes, functions, objects, and templates may be declared almost anywhere, may be declared any number of times, and must be declared before referring to them in any way. In a few weird cases, you can declare classes as you use them; won't cover that here.
Definitions
Classes may be defined at most once[1] per TU; this typically happens when you include a header for a particular class. Functions and objects must be defined once in exactly one TU; this typically happens when you implement them in a **.cpp* file. However, inline functions, including implicitly inline functions inside class definitions, may be defined in multiple TUs, but the definitions must be identical.
For practical purposes[2], templates (both class templates and function templates) are defined only in headers, and if you want to use a separate file, then use another header[3].
[1] Because of the at-most-once restriction, headers use include guards to prevent multiple inclusion and thus multiple definition errors.
[2] I won't cover the other possibilities here.
[3] Name it blahblah_detail.hpp, blahblah_private.hpp, or similar if you want to document that it's non-public.
Guidelines
So, while I'm sure everything above is all a big ball of mud so far, it's less than a page on what should take up a few chapters, so use it as a brief reference. Understanding the concepts above, however, is important. Using those, here's a short list of guidelines (but not absolute rules):
Always name headers consistently in a single project, such as **.h* for C and **.hpp* for C++.
Never include a file which is not a header.
Always name implementation files (which are going to be directly compiled) consistently, such as **.c* and **.cpp*.
Use a build system which can compile your source files automatically. make is the canonical example, but there are many alternatives. Keep it simple in simple cases. For example, make can be used its built-in rules and even without a makefile.
Use a build system which can generate header dependencies. Some compilers can generate this with command-line switches, such as -M, so you can make a surprisingly useful system easily.
Build Process
(Here's the tiny bit that answers your question, but you need most of the above in order to get here.)
When you build, the build system will then go through several steps, of which the important ones for this discussion are:
compile each implementation file as a TU, producing an object file (**.o*, **.obj*)
each is compiled independently of the others, which is why each TU needs declarations and definitions
link those files, along with libraries specified, into a single executable
I recommend you learn the rudiments of make, as it is popular, well-understood, and easy to get started with. However, it's an old system with several problems, and you'll want to switch to something else at some point.
Choosing a build system is almost a religious experience, like choosing an editor, except you'll have to work with more people (everyone working on the same project) and will likely be much more constrained by precedent and convention. You can use an IDE which handles the same details for you, but this has no real benefit from using a comprehensive build system instead, and you really should still know what it's doing under the hood.
File Templates
example.hpp
#ifndef EXAMPLE_INCLUDE_GUARD_60497EBE580B4F5292059C8705848F75
#define EXAMPLE_INCLUDE_GUARD_60497EBE580B4F5292059C8705848F75
// all project-specific macros for this project are prefixed "EXAMPLE_"
#include <ostream> // required headers/"modules"/libraries from the
#include <string> // stdlib, this project, and elsewhere
#include <vector>
namespace example { // main namespace for this project
template<class T>
struct TemplateExample { // for practical purposes, just put entire
void f() {} // definition of class and all methods in header
T data;
};
struct FooBar {
FooBar(); // declared
int size() const { return v.size(); } // defined (& implicitly inline)
private:
std::vector<TemplateExample<int> > v;
};
int main(std::vector<std::string> args); // declared
} // example::
#endif
example.cpp
#include "example.hpp" // include the headers "specific to" this implementation
// file first, helps make sure the header includes anything it needs (is
// independent)
#include <algorithm> // anything additional not included by the header
#include <iostream>
namespace example {
FooBar::FooBar() : v(42) {} // define ctor
int main(std::vector<std::string> args) { // define function
using namespace std; // use inside function scope, if desired, is always okay
// but using outside function scope can be problematic
cout << "doing real work now...\n"; // no std:: needed here
return 42;
}
} // example::
main.cpp
#include <iostream>
#include "example.hpp"
int main(int argc, char const** argv) try {
// do any global initialization before real main
return example::main(std::vector<std::string>(argv, argv + argc));
}
catch (std::exception& e) {
std::cerr << "[uncaught exception: " << e.what() << "]\n";
return 1; // or EXIT_FAILURE, etc.
}
catch (...) {
std::cerr << "[unknown uncaught exception]\n";
return 1; // or EXIT_FAILURE, etc.
}
This is called separate compilation model. You include class declarations into each module where they are needed, but define them only once.
In addition to hiding implementation details in cpp files (check other replies), you can additionally hide structure details by class forward declaration.
class FooPrivate;
class Foo
{
public:
// public stuff goes here
private:
FooPrivate *foo_private;
};
The expression class FooPrivate says that FooPrivate is completely defined somewhere else (preferably in the same file where Foo's implementation resides, before Foo's stuff comes. This way you make sure that implementation details of Foo(Private) aren't exposed via the header file.
You needn't include .c or .cpp files - the compiler will compile them regardless whether they're #included in other files or not. However, the code in the .c/.cpp files is useless if the other files are unaware of the classes/methods/functions/global vars/whatever that's contained in them. And that's where headers come into play. In the headers, you only put declarations, such as this one:
//myfile.hpp
class MyClass {
public:
MyClass (void);
void myMethod (void);
static int myStaticVar;
private:
int myPrivateVar;
};
Now, all .c/.cpp files that will #include "myfile.hpp" will be able to create instances of MyClass, operate on myStaticVar and call MyClass::myMethod(), even though there's no actual implementation here! See?
The implementation (the actual code) goes into myfile.cpp, where you tell the compiler what all your stuff does:
//myfile.cpp
int MyClass::myStaticVar = 0;
MyClass::MyClass (void) {
myPrivateVar = 0;
}
void MyClass::myMethod (void) {
myPrivateVar++;
}
You never include this file anywhere, it's absolutely not necessary.
A tip: create a main.hpp (or main.h, if you prefer - makes no difference) file and put all the #includes there. Each .c/.cpp file will then only need to have have this line: #include "main.hpp". This is enough to have access to all classes, methods etc. you declared in your entire project :).
You should not include a source file (.c or .cpp). Instead you should include the corresponding header file(.h) containing the declarations. The source files needs to be compiled separately and linked together to get the final executable.
Cpp files should be defined in your compiler script to be compiled as object files.
What ide are you using?
I am going to assume you are compiling with gcc, so here is the command to compile two .cpp files into one executable
gcc -o myclasses.out myclass.cpp myotherclass.cpp
You should only use #include to include class definitions, not the implentation
One thing you will want to watch out for when including you class declarations from a .h/.hpp is make sure it only ever gets included once. If you don't do this you will get some possibly cryptic compiler errors that will drive you up the wall.
To do this you need to tell the compiler, using a #define, to include the file only if the #define does not already exist.
For example (MyClass.h):
#ifndef MYCLASS_H
#define MYCLASS_H
class MyClass
{
// Memebers and methods
}
#endif
// End of file
This will guarantee your class declaration only gets included once even if you have it included in many different .cpp files.