`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.