Related
What are undeclared identifier errors? What are common causes and how do I fix them?
Example error texts:
For the Visual Studio compiler: error C2065: 'cout' : undeclared identifier
For the GCC compiler: 'cout' undeclared (first use in this function)
They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:
Missing header
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
To fix it, we must include the header:
#include <iostream>
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
If you wrote the header and included it correctly, the header may contain the wrong include guard.
To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.
Misspelled variable
Another common source of beginner's error occur when you misspelled a variable:
int main() {
int aComplicatedName;
AComplicatedName = 1; /* mind the uppercase A */
return 0;
}
Incorrect scope
For example, this code would give an error, because you need to use std::string:
#include <string>
int main() {
std::string s1 = "Hello"; // Correct.
string s2 = "world"; // WRONG - would give error.
}
Use before declaration
void f() { g(); }
void g() { }
g has not been declared before its first use. To fix it, either move the definition of g before f:
void g() { }
void f() { g(); }
Or add a declaration of g before f:
void g(); // declaration
void f() { g(); }
void g() { } // definition
stdafx.h not on top (VS-specific)
This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:
#include <iostream>
#include "stdafx.h"
The #include <iostream> would be ignored. You need to move it below:
#include "stdafx.h"
#include <iostream>
Feel free to edit this answer.
Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.
The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.
If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.
Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.
In C and C++ all names have to be declared before they are used. If you try to use the name of a variable or a function that hasn't been declared you will get an "undeclared identifier" error.
However, functions are a special case in C (and in C only) in that you don't have to declare them first. The C compiler will the assume the function exists with the number and type of arguments as in the call. If the actual function definition does not match that you will get another error. This special case for functions does not exist in C++.
You fix these kind of errors by making sure that functions and variables are declared before they are used. In the case of printf you need to include the header file <stdio.h> (or <cstdio> in C++).
For standard functions, I recommend you check e.g. this reference site, and search for the functions you want to use. The documentation for each function tells you what header file you need.
I had the same problem with a custom class, which was defined in a namespace. I tried to use the class without the namespace, causing the compiler error "identifier "MyClass" is undefined".
Adding
using namespace <MyNamespace>
or using the class like
MyNamespace::MyClass myClass;
solved the problem.
These error meassages
1.For the Visual Studio compiler: error C2065: 'printf' : undeclared identifier
2.For the GCC compiler: `printf' undeclared (first use in this function)
mean that you use name printf but the compiler does not see where the name was declared and accordingly does not know what it means.
Any name used in a program shall be declared before its using. The compiler has to know what the name denotes.
In this particular case the compiler does not see the declaration of name printf . As we know (but not the compiler) it is the name of standard C function declared in header <stdio.h> in C or in header <cstdio> in C++ and placed in standard (std::) and global (::) (not necessarily) name spaces.
So before using this function we have to provide its name declaration to the compiler by including corresponding headers.
For example
C:
#include <stdio.h>
int main( void )
{
printf( "Hello World\n" );
}
C++:
#include <cstdio>
int main()
{
std::printf( "Hello World\n" );
// or printf( "Hello World\n" );
// or ::printf( "Hello World\n" );
}
Sometimes the reason of such an error is a simple typo. For example let's assume that you defined function PrintHello
void PrintHello()
{
std::printf( "Hello World\n" );
}
but in main you made a typo and instead of PrintHello you typed printHello with lower case letter 'p'.
#include <cstdio>
void PrintHello()
{
std::printf( "Hello World\n" );
}
int main()
{
printHello();
}
In this case the compiler will issue such an error because it does not see the declaration of name printHello. PrintHello and printHello are two different names one of which was declared and other was not declared but used in the body of main
It happened to me when the auto formatter in a visual studio project sorted my includes after which the pre compiled header was not the first include anymore.
In other words. If you have any of these:
#include "pch.h"
or
#include <stdio.h>
or
#include <iostream>
#include "stdafx.h"
Put it at the start of your file.
If your clang formatter is sorting the files automatically, try putting an enter after the pre compiled header. If it is on IBS_Preserve it will sort each #include block separately.
#include "pch.h" // must be first
#include "bar.h" // next block
#include "baz.h"
#include "foo.h"
More info at
Compiler Error C2065
A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. In C++ all names have to be declared before they are used. If you try to use the name of a such that hasn't been declared you will get an "undeclared identifier" compile-error.
According to the documentation, the declaration of printf() is in cstdio i.e. you have to include it, before using the function.
Another possible situation: accessing parent (a template class) member in a template class.
Fix method: using the parent class member by its full name (by prefixing this-> or parentClassName:: to the name of the member).
see: templates: parent class member variables not visible in inherited class
one more case where this issue can occur,
if(a==b)
double c;
getValue(c);
here, the value is declared in a condition and then used outside it.
It is like Using the function without declaring it. header file will contain the
function printf(). Include the header file in your program this is the solution for that.
Some user defined functions may also through error when not declared before using it. If
it is used globally no probs.
Most of the time, if you are very sure you imported the library in question, Visual Studio will guide you with IntelliSense.
Here is what worked for me:
Make sure that #include "stdafx.h" is declared first, that is, at the top of all of your includes.
Every undeclared variable in c error comes because the compiler is not able to find it in the project. One can include the external (header) file of the library in which the variable is defined. Hence in your question, you require <stdio.h>, that is a standard input output file, which describes printf(), functionality.
According to the documentation, the declaration of fprintf() is in i.e. you have to include it, before using the function.
Check if you are importing the same packages in your .m and in your .h
Example given: I had this very problem with the init method and it was caused by missing the "#import " on the .m file
What are undeclared identifier errors? What are common causes and how do I fix them?
Example error texts:
For the Visual Studio compiler: error C2065: 'cout' : undeclared identifier
For the GCC compiler: 'cout' undeclared (first use in this function)
They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:
Missing header
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
To fix it, we must include the header:
#include <iostream>
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
If you wrote the header and included it correctly, the header may contain the wrong include guard.
To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.
Misspelled variable
Another common source of beginner's error occur when you misspelled a variable:
int main() {
int aComplicatedName;
AComplicatedName = 1; /* mind the uppercase A */
return 0;
}
Incorrect scope
For example, this code would give an error, because you need to use std::string:
#include <string>
int main() {
std::string s1 = "Hello"; // Correct.
string s2 = "world"; // WRONG - would give error.
}
Use before declaration
void f() { g(); }
void g() { }
g has not been declared before its first use. To fix it, either move the definition of g before f:
void g() { }
void f() { g(); }
Or add a declaration of g before f:
void g(); // declaration
void f() { g(); }
void g() { } // definition
stdafx.h not on top (VS-specific)
This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:
#include <iostream>
#include "stdafx.h"
The #include <iostream> would be ignored. You need to move it below:
#include "stdafx.h"
#include <iostream>
Feel free to edit this answer.
Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.
The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.
If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.
Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.
In C and C++ all names have to be declared before they are used. If you try to use the name of a variable or a function that hasn't been declared you will get an "undeclared identifier" error.
However, functions are a special case in C (and in C only) in that you don't have to declare them first. The C compiler will the assume the function exists with the number and type of arguments as in the call. If the actual function definition does not match that you will get another error. This special case for functions does not exist in C++.
You fix these kind of errors by making sure that functions and variables are declared before they are used. In the case of printf you need to include the header file <stdio.h> (or <cstdio> in C++).
For standard functions, I recommend you check e.g. this reference site, and search for the functions you want to use. The documentation for each function tells you what header file you need.
I had the same problem with a custom class, which was defined in a namespace. I tried to use the class without the namespace, causing the compiler error "identifier "MyClass" is undefined".
Adding
using namespace <MyNamespace>
or using the class like
MyNamespace::MyClass myClass;
solved the problem.
These error meassages
1.For the Visual Studio compiler: error C2065: 'printf' : undeclared identifier
2.For the GCC compiler: `printf' undeclared (first use in this function)
mean that you use name printf but the compiler does not see where the name was declared and accordingly does not know what it means.
Any name used in a program shall be declared before its using. The compiler has to know what the name denotes.
In this particular case the compiler does not see the declaration of name printf . As we know (but not the compiler) it is the name of standard C function declared in header <stdio.h> in C or in header <cstdio> in C++ and placed in standard (std::) and global (::) (not necessarily) name spaces.
So before using this function we have to provide its name declaration to the compiler by including corresponding headers.
For example
C:
#include <stdio.h>
int main( void )
{
printf( "Hello World\n" );
}
C++:
#include <cstdio>
int main()
{
std::printf( "Hello World\n" );
// or printf( "Hello World\n" );
// or ::printf( "Hello World\n" );
}
Sometimes the reason of such an error is a simple typo. For example let's assume that you defined function PrintHello
void PrintHello()
{
std::printf( "Hello World\n" );
}
but in main you made a typo and instead of PrintHello you typed printHello with lower case letter 'p'.
#include <cstdio>
void PrintHello()
{
std::printf( "Hello World\n" );
}
int main()
{
printHello();
}
In this case the compiler will issue such an error because it does not see the declaration of name printHello. PrintHello and printHello are two different names one of which was declared and other was not declared but used in the body of main
It happened to me when the auto formatter in a visual studio project sorted my includes after which the pre compiled header was not the first include anymore.
In other words. If you have any of these:
#include "pch.h"
or
#include <stdio.h>
or
#include <iostream>
#include "stdafx.h"
Put it at the start of your file.
If your clang formatter is sorting the files automatically, try putting an enter after the pre compiled header. If it is on IBS_Preserve it will sort each #include block separately.
#include "pch.h" // must be first
#include "bar.h" // next block
#include "baz.h"
#include "foo.h"
More info at
Compiler Error C2065
A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. In C++ all names have to be declared before they are used. If you try to use the name of a such that hasn't been declared you will get an "undeclared identifier" compile-error.
According to the documentation, the declaration of printf() is in cstdio i.e. you have to include it, before using the function.
Another possible situation: accessing parent (a template class) member in a template class.
Fix method: using the parent class member by its full name (by prefixing this-> or parentClassName:: to the name of the member).
see: templates: parent class member variables not visible in inherited class
one more case where this issue can occur,
if(a==b)
double c;
getValue(c);
here, the value is declared in a condition and then used outside it.
It is like Using the function without declaring it. header file will contain the
function printf(). Include the header file in your program this is the solution for that.
Some user defined functions may also through error when not declared before using it. If
it is used globally no probs.
Most of the time, if you are very sure you imported the library in question, Visual Studio will guide you with IntelliSense.
Here is what worked for me:
Make sure that #include "stdafx.h" is declared first, that is, at the top of all of your includes.
Every undeclared variable in c error comes because the compiler is not able to find it in the project. One can include the external (header) file of the library in which the variable is defined. Hence in your question, you require <stdio.h>, that is a standard input output file, which describes printf(), functionality.
According to the documentation, the declaration of fprintf() is in i.e. you have to include it, before using the function.
Check if you are importing the same packages in your .m and in your .h
Example given: I had this very problem with the init method and it was caused by missing the "#import " on the .m file
In a .cpp file I implement a bubble_sort algorithm. Inside the bubble_sort function I use swap(a, b). However I don't specify "using name space std" in this cpp file. (Actually there is no header in this .cpp file) I still could complile the program without any warning or error. I know that if I want to use some customary function in that bubble_sort function, I need to at lease declare that function in the same file. But I don't understand why I don't need to do any thing for the swap. Below is my cpp file:
bubble_sort.cpp
template <typename Type>
void bubble_sort(Type* originarray, int lengthofarray)
{
int ii=lengthofarray-1;
while(ii>0)
{
for (int jj=0;jj<ii;jj++)
{
if (originarray[jj]>originarray[jj+1])
swap(originarray[jj],originarray[jj+1]);
}
ii--;
}
}
First, to properly test template code (at least with MSVC), you should instantiate it.
Moreover, if you try your code with some class that is in the std namespace, std::swap() can be picked by the compiler via ADL (Koenig) lookup.
What are undeclared identifier errors? What are common causes and how do I fix them?
Example error texts:
For the Visual Studio compiler: error C2065: 'cout' : undeclared identifier
For the GCC compiler: 'cout' undeclared (first use in this function)
They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:
Missing header
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
To fix it, we must include the header:
#include <iostream>
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
If you wrote the header and included it correctly, the header may contain the wrong include guard.
To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.
Misspelled variable
Another common source of beginner's error occur when you misspelled a variable:
int main() {
int aComplicatedName;
AComplicatedName = 1; /* mind the uppercase A */
return 0;
}
Incorrect scope
For example, this code would give an error, because you need to use std::string:
#include <string>
int main() {
std::string s1 = "Hello"; // Correct.
string s2 = "world"; // WRONG - would give error.
}
Use before declaration
void f() { g(); }
void g() { }
g has not been declared before its first use. To fix it, either move the definition of g before f:
void g() { }
void f() { g(); }
Or add a declaration of g before f:
void g(); // declaration
void f() { g(); }
void g() { } // definition
stdafx.h not on top (VS-specific)
This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:
#include <iostream>
#include "stdafx.h"
The #include <iostream> would be ignored. You need to move it below:
#include "stdafx.h"
#include <iostream>
Feel free to edit this answer.
Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.
The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.
If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.
Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.
In C and C++ all names have to be declared before they are used. If you try to use the name of a variable or a function that hasn't been declared you will get an "undeclared identifier" error.
However, functions are a special case in C (and in C only) in that you don't have to declare them first. The C compiler will the assume the function exists with the number and type of arguments as in the call. If the actual function definition does not match that you will get another error. This special case for functions does not exist in C++.
You fix these kind of errors by making sure that functions and variables are declared before they are used. In the case of printf you need to include the header file <stdio.h> (or <cstdio> in C++).
For standard functions, I recommend you check e.g. this reference site, and search for the functions you want to use. The documentation for each function tells you what header file you need.
I had the same problem with a custom class, which was defined in a namespace. I tried to use the class without the namespace, causing the compiler error "identifier "MyClass" is undefined".
Adding
using namespace <MyNamespace>
or using the class like
MyNamespace::MyClass myClass;
solved the problem.
These error meassages
1.For the Visual Studio compiler: error C2065: 'printf' : undeclared identifier
2.For the GCC compiler: `printf' undeclared (first use in this function)
mean that you use name printf but the compiler does not see where the name was declared and accordingly does not know what it means.
Any name used in a program shall be declared before its using. The compiler has to know what the name denotes.
In this particular case the compiler does not see the declaration of name printf . As we know (but not the compiler) it is the name of standard C function declared in header <stdio.h> in C or in header <cstdio> in C++ and placed in standard (std::) and global (::) (not necessarily) name spaces.
So before using this function we have to provide its name declaration to the compiler by including corresponding headers.
For example
C:
#include <stdio.h>
int main( void )
{
printf( "Hello World\n" );
}
C++:
#include <cstdio>
int main()
{
std::printf( "Hello World\n" );
// or printf( "Hello World\n" );
// or ::printf( "Hello World\n" );
}
Sometimes the reason of such an error is a simple typo. For example let's assume that you defined function PrintHello
void PrintHello()
{
std::printf( "Hello World\n" );
}
but in main you made a typo and instead of PrintHello you typed printHello with lower case letter 'p'.
#include <cstdio>
void PrintHello()
{
std::printf( "Hello World\n" );
}
int main()
{
printHello();
}
In this case the compiler will issue such an error because it does not see the declaration of name printHello. PrintHello and printHello are two different names one of which was declared and other was not declared but used in the body of main
It happened to me when the auto formatter in a visual studio project sorted my includes after which the pre compiled header was not the first include anymore.
In other words. If you have any of these:
#include "pch.h"
or
#include <stdio.h>
or
#include <iostream>
#include "stdafx.h"
Put it at the start of your file.
If your clang formatter is sorting the files automatically, try putting an enter after the pre compiled header. If it is on IBS_Preserve it will sort each #include block separately.
#include "pch.h" // must be first
#include "bar.h" // next block
#include "baz.h"
#include "foo.h"
More info at
Compiler Error C2065
A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. In C++ all names have to be declared before they are used. If you try to use the name of a such that hasn't been declared you will get an "undeclared identifier" compile-error.
According to the documentation, the declaration of printf() is in cstdio i.e. you have to include it, before using the function.
Another possible situation: accessing parent (a template class) member in a template class.
Fix method: using the parent class member by its full name (by prefixing this-> or parentClassName:: to the name of the member).
see: templates: parent class member variables not visible in inherited class
one more case where this issue can occur,
if(a==b)
double c;
getValue(c);
here, the value is declared in a condition and then used outside it.
It is like Using the function without declaring it. header file will contain the
function printf(). Include the header file in your program this is the solution for that.
Some user defined functions may also through error when not declared before using it. If
it is used globally no probs.
Most of the time, if you are very sure you imported the library in question, Visual Studio will guide you with IntelliSense.
Here is what worked for me:
Make sure that #include "stdafx.h" is declared first, that is, at the top of all of your includes.
Every undeclared variable in c error comes because the compiler is not able to find it in the project. One can include the external (header) file of the library in which the variable is defined. Hence in your question, you require <stdio.h>, that is a standard input output file, which describes printf(), functionality.
According to the documentation, the declaration of fprintf() is in i.e. you have to include it, before using the function.
Check if you are importing the same packages in your .m and in your .h
Example given: I had this very problem with the init method and it was caused by missing the "#import " on the .m file
I was writing a program of function overloading in Visual C++ 2010 .
Following is my code
// overload.cpp : Defines the entry point for the console application.
#include<Windows.h>
#include<iostream>
#include<conio.h>
using namespace std;
//abs is overloaded in 3 types
int abs(int i);
double abs(double d);
long abs(long f);
void main()
{
cout<<abs(-10)<<"\n";
cout<<abs(-11.0)<<"\n";
cout<<abs(-9L)<<"\n";
getch();
}
int abs(int i)
{
cout<<"using integer abs()\n";
return i>0? -i:i;
}
double abs(double d)
{
cout<<"using double abs()\n";
return d>0? -d:d;
}
long abs (long l)
{
cout<<"using long abs()\n";
return l>0?-l:l;
}
I am having problems in double abs and long abs function that
1>c:\users\abc\documents\visual studio 2010\projects\overload\overload\overload.cpp(22): error C2084: function 'double abs(double)' already has a body
1>c:\users\abc\documents\visual studio 2010\projects\overload\overload\overload.cpp(26): error C2084: function 'long abs(long)' already has a body
Why this problem is coming?
I have changed the compilation from c to c++
but recently I ran an other program for overloading,it worked.I don't know how? here is the code.
#include<iostream>
#include<cstdio>
#include<conio.h>
#include<cstring>
using namespace std;
void stradd(char*s1,char*s2);
void stradd(char*s1,int i);
void main()
{
char str[80];
strcpy(str,"hello");
stradd(str,"there");
cout<<str<<"\n";
getch();
}
//concatenate a string with a "stringized "integer
void stradd(char*s1,int i)
{
char temp[80];
sprintf(temp,"%d",i);
strcat(s1,temp);
}
//concatenate 2 strings
void stradd(char*s1,char *s2)
{
strcat(s1,s2);
}
and output is hellothere
Your problem comes from a header in which abs is declared for some types such as double. You're not allowed to have to functions with exactly the same header (that is, same return type, same name, same list of parameters, same qualifiers such as const).
There are two ways of avoiding this:
Use the standard library: std::abs is good, you don't need to implement it yourself
Naming the method absoluteValue or myAbs or whatever you like, but not abs
A third way, namely removing using namespace std does not work according to your comment. This is because you include Windows.h. This itself includes a bunch of headers, probably including math.h. This gives a method called abs in the global namespace. Better don't include Windows.h and include cmath if you need to. Then, abs is only declared in namespace std, hence you can call it with std::abs and is different from abs.
When overload resolution cannot select one function as the unique best match, the call is ambiguous. An ambiguous call produces a compilation error.
In std there is already an abs() with the following signature:
int abs (int n);
So while you try to overload it with double and long it results in ambiguity for the compiler.
If you're a beginner learning about coding i suggest you to use function names not defined in libraries (at least the ones you have included).
stefan have already given the solution to it:
Remove using namespace std; and explicitly write std::cout
OR
Re name your function to absoluteValue or something else
OR
Use explicit namespaces in function declaration and calls. (Not tested, though it should work)
Put your function inside a class or namespace.
Maybe this would provide you with a little insight (From SO).
EDIT:
The second question's overloaded functions stradd() is not defined in any other library. That is why no Compilation Errors. The following function signature in your code will result an error: char * strcat ( char * destination, const char * source )
Your primary problem is that you use global namespace. Just declare your function in your own namespace and all name collisions will be gone.
Let me explain why you're getting those compile-time errors.
Somewhere in the headers you included there are double abs(double) and long abs(long) functions. Then you're creating functions with the same signatures by your own. So compiler just don't know what to use when you'll call one of them - there are 2 pairs of equal functions. So it refuses to compile that, and you're getting those errors.
So you have 2 choices - hope that every time you'll want to create a function you will choose an unique name, or just create a namespace and your function names should be unique only to another functions in your namespace.
And it's not about overloading - void func(int i) overloads void func(float f), but void func(int i) overrides void func(int i). You can override superclass member functions in subclasses, but you cannot override standalone functions like abs().
just change abs function name with another.
abs() is a keyword therefore it is showing errors.