`does not name a type` error with `namespace std;` and files

Question

compiling the code below with g++ main.cpp functions.cpp -o run gives me the error error: 'vector' does not name a type . Declaring namespace at the top of main.cpp usually works across all .cpp files for me.

main.cpp

using namespace std;

#include "functions.h"

main () {}

functions.h

#include <vector>

functions.cpp

#include "functions.h"
vector <int> x;

EDIT: I appreciate the fact all responders know what their talking about, but this normally works for me. Would the use of a makefile have any bearing on that? something else that I might be missing?

Answer 1

Yes but in this example functions.cpp has not seen using namespace std since you only wrote that in main.cpp .

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Don't add using namespace std to functions.h , use std:: to qualify types. Adding a using.. imposes an unnecessary burden on the user of your header.

Answer 2

You need to qualify the namespace:

#include "functions.h"
std::vector<int> x;

You have a using namespace std in main.cpp , and it cannot be seen by functions.cpp . That is the root of the problem.

In general, you should avoid using namespace std , specially in headers. And if you really must include it in main , put it after all the headers.

Answer 3

You imported the std namespace only in main.cpp , not in functions.cpp .

You have to qualify your use - std::vector in the second file, or use the using directive:

//functions.cpp
#include "functions.h"
std::vector <int> x;   // preferred

or

//functions.cpp
#include "functions.h"
using namespace std;
vector <int> x;

or (bonus)

//functions.cpp
#include "functions.h"
using std::vector;
vector <int> x;

Declaring namespace at the top of main.cpp usually works across all .cpp files for me.

You have a really flawed compiler then. using directives shouldn't influence translation units that don't have direct visibility over the directive.

Answer 4

You using namespace std is local to main.cpp only. You need to use

 std::vector<int> x;

in your source file functions.cpp