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.
Related
I was just wondering where is it necessary/right to include a specific header file according to the example below. Let's assume I have a definition of an exception class:
//exc.hpp
#ifndef EXC_H
#define EXC_H
class MyException : public exception {
};
#endif /* EXC_H */
Then I have another class definition throwing such exception:
//a.cpp
void SomeClass::someMethod(void) {
throw MyException(...);
}
And having another file handling that exception, e.g.:
//main.cpp
#include "a.hpp"
int main() {
...
catch(MyException & e) { ... }
}
So my question is, where should I place #include "exc.hpp"? Just to a.hpp, or both a.hpp and main.cpp?
And when it comes to makefile... How should be the targets specified within such file?
Every translation unit that throws or catches exceptions of that type will need to be able to see its definition.
Loosely speaking, that means every .cpp file containing a throw or catch relating to that exception must include your .hpp file.
That's even if you only catch by reference and never inspect the exception object, which is not the case in other areas of C++ (where a forward declaration will do).
Makefiles are unrelated.
Both the file with implementation (a.cpp) and all files the use the class (main.cpp) should have the #include.
a.cpp without the include must not compile at all.
You need to include the .hpp file in both main.cpp and a.cpp. The purpose of the #ifndef sequence is to prevent accidental multiple inclusion (through indirect #includes). There is also a #pragma once directive in MS compilers that does the same thing.
The compiler figures out what .h/.hpp files to read based on the #includes in the .cpp files; the make file is not involved.
Remember that the compiler processes each source file independently, and doesn't remember anything from the source file once it's done processing. Even if you list several source files on a single compiler command-line.
You have a header-file that defines a type. Naturally, you must #include the header file in every source file where you need that type to be defined. (The compiler will not remember having seen the types when processing earlier source files.)
It might be tempting to #include headers within other headers, just so you don't have to #include so many things within the .c or .cpp files, but this should be avoided to the degree possible. It produces what is known as "header coupling", and it makes code that is hard to re-use later on other projects.
There's also a fine point hiding in what I said above: "where you need that type to be defined". There are two very specific concepts in C and C++ related to variables:
declaration -- when you make the compiler aware that a type exists, and
definition -- where you tell the compiler the details of the type.
You need to #include your header wherever you need the definition. I.e., when you intend to instantiate an object of the type, define members of the type in another struct or class, or call one of its methods (assuming C++). If instead, you only want to store a reference to objects of the type without creating or using them, a declaration will suffice, and you can just forward-declare the class. I.e.,
class MyException;
void setFileNotFoundExceptionObject(const MyException *exc) { ... }
Many APIs are designed specifically around only using pointers or references to objects so that the API headers only need forward delcarations of the types, and not the complete definitions (This keeps the internal members of the objects hidden to prevent developers from abusing them.)
When I started learning C++ I learned that header files should typically be #included in other header files. Just now I had someone tell me that I should include a specific header file in my .cpp file to avoid header include creep.
Could someone tell me what exactly that is, why it is a problem and maybe point me to some documentation on when I would want to include a file in another header and when I should include one in a .cpp file?
The "creep" refers to the inclusion of one header including many others. This has some unwanted consequences:
a tendency towards every source file indirectly including every header, so that a change to any header requires everything to be recompiled;
more likelihood of circular dependencies causing heartache.
You can often avoid including one header from another by just declaring the classes you need, rather than including the header that gives the complete definition. Such an incomplete type can be used in various ways:
// Don't need this
//#include "thingy.h"
// just this
class thingy;
class whatsit {
// You can declare pointers and references
thingy * p;
thingy & r;
// You can declare static member variables (and external globals)
// They will need the header in the source file that defines them
static thingy s;
// You can declare (but not define) functions with incomplete
// parameter or return types
thingy f(thingy);
};
Some things do require the complete definition:
// Do need this
#include "thingy.h"
// Needed for inheritance
class whatsit : thingy {
// Needed for non-static member variables
thingy t;
// Needed to do many things like copying, accessing members, etc
thingy f() {return t;}
};
Could someone tell me what exactly [include creep] is
It's not a programming term, but interpreting it in an Engish-language context would imply that it's the introduction of #include statements that are not necessary.
why it is a problem
Because compiling code takes time. So compiling code that's not necessary takes unnecessary time.
and maybe point me to some documentation on when I would want to include a file in another header and when I should include one in a .cpp file?
If your header requires the definition of a type, you will need to include the header that defines that type.
#include "type.h"
struct TypeHolder
{
Type t;
// ^^^^ this value type requires the definition to know its size.
}
If your header only requires the declaration of a type, the definition is unnecessary, and so is the include.
class Type;
struct TypeHolder
{
Type * t;
// ^^^^^^ this pointer type has the already-known size of a pointer,
// so the definition is not required.
}
As an anecdote that supports the value of this practice, I was once put on a project whose codebase required ONE HOUR to fully compile. And changing a header often incurred most or all of that hour for the next compilation.
Adding forward-declarations to the headers where applicable instead of includes immediately reduced full compilation time to 16 minutes, and changing a header often didn't require a full rebuild.
Well they were probably referring to a couple of things ("include creep" is not a term I've ever heard before). If you, as an extreme rule, only include headers from headers (aside from one matching header per source file):
Compilation time increases as many headers must be compiled for all source files.
Unnecessary dependencies increase, e.g. with a dependency based build system, modifying one header may cause unnecessary recompilation of many other files, increasing compile time.
Increased possibility of namespace pollution.
Circular dependencies become an issue (two headers that need to include each other).
Other more advanced dependency-related topics (for example, this defeats the purpose of opaque pointers, which causes many design issues in certain types of applications).
As a general rule of thumb, include the bare minimum number of things from a header required to compile only the code in that header (that is, enough to allow the header to compile if it is included by itself, but no more than that). This will combat all of the above issues.
For example, if you have a source file for a class that uses std::list in the code, but the class itself has no members or function parameters that use std::list, there's no reason to #include <list> from the class's header, as nothing in the class's header uses std::list. Do it from the source file, where you actually need it, instead of in the header, where everything that uses the class also has to compile <list> unnecessarily.
Another common technique is forward declaring pointer types. This is the only real way to combat circular dependency issues, and has some of the compilation time benefits listed above as well. For example, if two classes refer to each other but only via pointers:
// A.h
class B; // forward declaration instead of #include "B.h"
class A {
B *b;
}
// B.h
class A; // forward declaration instead of #include "A.h"
class B {
A *a;
}
Then include the headers for each in the source files, where the definitions are actually needed.
Your header files should include all headers they need to compile when included alone (or first) in a source file. You may in many cases allow the header to compile by using forward declarations, but you should never rely on someone including a specific header before they include yours.
theres compie time to think of, but there's also dependancies. if A.h includes B.h and viceversa, code won't compile. You can get around this by forward referancing your class, then including the header in the cpp
A.h
class B;
class A
{
public:
void CreateNewB():
private:
B* m_b;
};
A.cpp
#include "B.h"
A::CreateNewB()
{
m_b = new B;
}
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.
I am using GCC C++ 11 in CodeBlocks IDE. I have been trying to reuse classes that I wrote by putting them into a header file but it doesn't work. I have looked into many books but none has detailed information on making C++ code reusable.
There are a couple concepts that C++ uses that I think you're missing:
The difference between a declaration and a definition
#include statements
Linking against other libraries
In general, one reuses code in C++ by Declaring a function in a header file (.h), Defining it in a source file (.cpp). The header file ("foo.h") is then included in both the source file (foo.cpp) and any other file you want to use something declared in it using and preprocessor include directive #include "foo.h". Finally if the source file in which the functions declared in the header file are defined is not a part of the source for the library or executable that you're compiling, you will need to link against the library in which it was built.
Here's a simple example that assumes that you don't need to link against an external library and that all files are in the same directory:
foo.h:
The class Foo is declared along with a member function foobar and a private member variable barM. In this file we're telling the world that there is a class named Foo, it's constructor and destructor are public, it has a member function named fooBar that returns an integer and it also has a private member variable named barM.
class Foo
{
public:
Foo();
~Foo();
int fooBar();
private:
int barM;
};
foo.cpp
The individual member functions for our class Foo are defined, we implement the things we declared in the header file. Notice the include statement at the top
#include "foo.h"
Foo::Foo()
{
barM = 10;
}
Foo::~Foo()
{
}
int Foo::fooBar()
{
return barM;
}
main.cpp
We use our class in a different file, again the header file is included at the top.
#include <stdio.h>
#include "foo.h"
int main(int argc, char *argv[])
{
Foo flub;
std::cout << "flub's fooBar is: " << flub.fooBar() << std::endl();
return 0;
}
The expected output from this would be:
flub's fooBar is 10.
As a general note, I haven't compiled this code, but it should be enough to give you a basic example of the ideas of declarations, definitions, and include statements.
Seeing as you're coming from Java, I'm actually betting that you got all of that already, the hard part is actually using code from a different c++ library, which is akin to Java packages. Setting this up requires exporting the symbols you desired to use in a different library. The way to do this is compiler specific, but is generally accomplished by defining a Macro in a different header file and then using that macro in the declaration of the item you'd like to export. For gcc, see reference GNU Reference Manual.
To extend the above example you create another header file: fooLibExport.h
#if BUILDING_LIBFOO && HAVE_VISIBILITY
#define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
#elif BUILDING_LIBFOO && defined _MSC_VER
#define LIBFOO_DLL_EXPORTED __declspec(dllexport)
#elif defined _MSC_VER
#define LIBFOO_DLL_EXPORTED __declspec(dllimport)
#else
#define LIBFOO_DLL_EXPORTED
#endif
foo.h would then be changed to:
#include "fooLibExport.h"
class LIBFOO_DLL_EXPORTED Foo
{
public:
Foo();
~Foo();
int fooBar();
private:
int barM;
};
Finally you'll need to link against the library that Foo was built into. Again this is compiler specific. At this point we're through the setting up of header files for exporting symbols from a C++ library so that functions defined in one library can be used in another. I'm going assume that you can follow the reference material for setting up the GCC compiler for the rest of the process. I've tried to bold the key words that should help refine your searches.
One final note about #include statements, the actual argument isn't just the filename, its the path, relative or absolute, to the file in question. So if the header file isn't in the same file as the file you're trying to include it in, you'll need to use the appropriate path to the file.
Code re-usability casts its net wide in C++ terminology. Please be specific what do you mean by it.C and C++ programming language features usually considered to be relevant to code reuse could be :-
functions, defined types, macros, composition, generics, overloaded functions and operators, and polymorphism.
EDITED IN RESPONSE TO COMMENT:-
Then you have to use header files for putting all declarations which you can use in any file just by including this.
In C# or Java, classes are declared and defined at the same time. In C++, the norm is to do that separately. What if we write the whole class in one , say .cpp, file and include that in files that references to it, what kinds of bad thing technically would happen besides a lengthened compilation process?
If your implementation of MyClass is all in the header file MyClass.h then any file you needed to implement MyClass will be included whenever someone includes MyClass.h.
If you change any part of MyClass.h, even if it's trivial (such as adding a comment or even a space) then all files that include it will have to recompile, even if the interface hasn't changed.
Neither of these matters for toy projects, but as you noted, when you have a program that consists of hundreds (or thousands, etc.) of class files, the added compilation time alone makes it worthwhile to separate out implementation from interface.
For instance, if I have the following:
// MyClass.h
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include "Inventory.h"
class MyClass
{
public:
MyClass();
void processInventory(Inventory& inventory)
{
// Do something with each item in the inventory here
// that uses iostream, iomanip, sstream, and string
}
private:
// ...
};
It would more ideomatically be written as:
// MyClass.h
class Inventory;
class MyClass
{
public:
MyClass();
void processInventory(Inventory& inventory);
private:
// ...
};
// MyClass.cc
#include "MyClass.h"
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include "Inventory.h"
MyClass()::MyClass()
{
}
void MyClass()::processInventory(Inventory& inventory)
{
// Do something with each item in the inventory here
// that uses iostream, iomanip, sstream, and string
}
Notice: Including MyClass.h doesn't mean iostream, iomanip, sstream, string, or Inventory.h have to be parsed. Changing how processInventory works doesn't mean all files using MyClass.h have to be recompiled.
Notice how much easier it can be to figure out how to use MyClass now. Header files serve an important purpose: they show people how to use your class. With the modified MyClass.h it's easy to see the list of functions. If each function is defined in the header, then you can't look at just the list of functions. That makes it harder to figure out how to use the class.
You may break the one definition rule.
If you write this:
class foo
{
public:
void doit();
};
foo::doit() {}
and include that in multiple classes, you will have multiple definitions of foo::doit and your link will fail.
But if you make all your classes inline, either by defining them within the class declaration:
class foo
{
public:
void doit() {
}
};
or by explicitly making them inline:
class foo
{
public:
void doit();
};
inline void foo::doit() {}
then you can include that file as many times as you like.
The linker will see multiple definitions of the class's members when you try to combine multiple such objects. Thus, you won't be able to produce a binary from source files that include anything in more than one place.
Typically you separate the declaration and definition of a class. This allows you to use your class in different source files by simply including the declaration.
If you include a .cpp which has both declaration and definition into 2 different source files then that class will be doubly defined.
Each .cpp that the class is included into will compile fine into object files. However each class must have only 1 definition total or else you will not be able to link your object files together.
The most important thing to understand about #include contrasted with other languages importing methods, is that #include COPIES the contents of that file where the #include directive is placed. So declaring and defining a class in the same file will create three things:
Significantly increase your compile
times.
If your definitions are not inline
you will get linker errors, since
the compiler finds multiple
definitions to the same functions
That would expose the implementation
to the user, instead of only the interface.
That is why it is common practice to define large classes in separate files, and on some ocassions, really small classes with small implementations (like smart pointers) in one file(To also implicitly inline methods).
#Bill
I think it is important to underscore Bill's point:
Notice how much easier it can be to
figure out how to use MyClass now.
Header files serve an important
purpose: they show people how to use
your class.
the .h file being more or less the "public" doc to allow the understanding of how your class works in some ways conceptually--an Interface. Remember the source file should be thought of as proprietary. I remember learning a lot about how Unix worked in my early C/C++ days by reading header files. Also remember that inline function complexities should be no more than accessor's
A big reason for a class to be defined in a cpp-file is that it isn't needed publically, it is just a helper function (like e.g. a functor). Some people seem to be afraid to put the complete class in the cpp-file, while that just shows your intent of only using the class there.
Files are usually the atoms of your version control system - if you partition things sensibly into different files, then it becomes possible for a team of developers to check out only the parts they each need to work on. Put everything in one file and you can't do that.