Qt and namespace error with library

Question

I had a perfectly working project in Qt and I had to add an external library (*.h and *.cpp) to continue working. However, after adding those files to the project I suddenly have 48 errors and they start with the problem with a namespace...

Here is my .pro file:

    QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = ean
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    IntervalArithmetic.cpp

HEADERS  += mainwindow.h \
    IntervalArithmetic.h

FORMS    += mainwindow.ui

QMAKE_CXXFLAGS += -std=c++11

LIBS += -lmpfr
LIBS += -lgmp

Here is the beginning of my *.h file added:

#ifndef INTERVALARITHMETIC_H_
#define INTERVALARITHMETIC_H_
#include <iostream>
#include <string>
#include <sstream>
#include <exception>
#include <fenv.h>
#include <stdlib.h>
#include <stdint.h>
#include <mpfr.h>


using namespace std;

namespace intervalarth
{

struct interval
{
    long double a, b;
};


class IntervalArithmetic
{
public:
    IntervalArithmetic();
...

And here is the beginning of the corresponding *.cpp file:

#include "IntervalArithmetic.h"

#include <sstream>
#include <iomanip>
#include <stdexcept>
#include <cfenv>
#include <cstdlib>
#include <cstdint>
#include <climits>
#include <cmath>

#include "mpfr.h"

using namespace std;
using namespace IntervalArithmetic;
...

errors can be seen on this picture:

Could you please help me?

Answer 1

Your namespace is called namespace intervalarth , but you're using namespace IntervalArithmetic; which is not a namespace-name, as stated in the first error on the screenshot above.

You must unify the namespace-name between *.cpp and *.h files.