`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?

Solution 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.

Solution 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.

Solution 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.

Solution 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

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Comments

[Matt Munson]

I will change downvote to an upvote for anyone who adresses the fact that this usually works for me. Even your answers is only speculative.

[juanchopanza]

@MattMunson it cannot possibly work for you (in this particular case), because using namespace std is not seen in functions.cpp. And assuming that using namespace std can be seen somewhere can only lead to trouble.