Why do I need a c++ namespace specifier while using namespace?

Question

following class ADate is declared enclosed by namespace A
.h file

#ifndef ADATE_H
#define ADATE_H
namespace A{
 class ADate
 {
 public:
 static unsigned int daysInMonth[];
 private:
 int day;
 int month;
 int year;
 public:
 ADate(const unsigned int day, const unsigned int month, const unsigned int year);
 };
 bool isValidDate(const unsigned int day, const unsigned int month, const unsigned int year);
}
#endif // ADATE_H

.cpp file:

#include "adate.h"
using namespace A;
unsigned int ADate::daysInMonth[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };
ADate::ADate(const unsigned int day, const unsigned int month, const unsigned int year) :
 day{day},
 month{month},
 year{year}
{
 if(!isValidDate(day,month,year)){
 throw string{"invalid Date"};
 }
}
bool isValidDate(const unsigned int day, const unsigned int month, const unsigned int year)
{
 if(month < 1 || month > 12){
 return false;
 }
 if(day < 1 || day > ADate::daysInMonth[month-1]){
 return false;
 }
 if(year < 1979 || year > 2038){
 return false;
 }
 return true;
}

One should think that the code above can be compiled successfully. However, this is not that way, cause an undefined reference to `A::isValidDate(unsigned int, unsigned int, unsigned int)' occurs.

I don't understand why I have to use the namespace-specifier as prefix for the global function 'isValidDate'.

Can you explain why. Thank you

• NathanOliver: Please stop using using namespace std; in header files. Further reading: Why is "using namespace std" in C++ considered bad practice?
• LogicStuff: A recent question.
• Nicol Bolas: Why does ADate's constructor take regular ints, but isValidDate take unsigned ints? You're kinda begging for signed/unsigned issues.
• Unimportant: The prototype for function isValidDate is inside the namespace A in the header file, so it's really not global.
• Loki Astari: Time for a code review (once you fix this bug). Checkout codereview.stackexchange.com

Answer 1

Because of the namespace lookup rules.

See: http://en.cppreference.com/w/cpp/language/lookup

For variables, namespaces, classes, etc (apart from functions), name lookup must produce a single declaration in order for the program to compile.

For functions name lookup can associate multiple declarations (which are then resolved by argument comparison).

So:

bool isValidDate(const unsigned int day, const unsigned int month, const unsigned int year)
{
 // CODE
}

Is both a declaration and a definition. The namespace resolution did not need to resolve the name of the function to the A::isValidDate() function so it does not. Instead it adds another declaration into the current scope for isValidDate().

• wuarmin: Ok, thank you for the explanation. Just for completeness: the member methods don't need the specifier, because of the class specifier of the class which was declared within the namspace. Is that correct?
• Loki Astari: @PaceyW. Correct the class name must be exactly resolved to a specific class (if multiple classes match this is a compile time error). Since there is now an exact match to the class you can match the definition to the declaration.