include file error in C++

Question

Problem fixed! Thanks a lot for the constructive suggestions!

I am unable to figure out what is the mistake in the following code. Is there something wrong with the way I am doing includes?

// This is utils.h
#ifndef UTILS_H
#define UTILS_H

#include <iostream>
#include <fstream>
#include <stack>
#include <queue>
#include <vector>
#include <list>
#include <string>
#include <algorithm>

typedef pair<int,int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<vii> vvii;
typedef stack<int> si;
typedef queue<int> qi;

#define tr(c,i) for(typeof((c).begin()) i = (c).begin() ; i!=(c).end() ; ++i )
#define all(c) (c).begin(),(c).end()
#define cpresent(c,x) (find(all(c),x) != (c).end())

#endif

// ==============================================================
// Below is main.cpp

#include "utils.h"

int main() {
    vi v;
}

On compiling "g++ main.cpp" I get the following error message:

utils.h:13: error: expected initializer before ‘<’ token utils.h:14: error: expected initializer before ‘<’ token utils.h:15: error: expected initializer before ‘<’ token utils.h:16: error: expected initializer before ‘<’ token utils.h:17: error: expected initializer before ‘<’ token utils.h:18: error: expected initializer before ‘<’ token main1.cpp: In function ‘int main()’: main1.cpp:4: error: ‘vi’ was not declared in this scope main1.cpp:4: error: expected `;' before ‘v’

What is wrong with this code? The utils.h used to work fine some time back when I did not have the #ifndefs in it.

Answer 1

Those types (pair, stack, queue, vector, etc.) are in the std namespace. You either need to add using namespace std; at the top of your file (generally after all of the standard library includes) or fully qualify the type names by adding std:: in front of them.

Generally, it's better practice to fully qualify the type names than to use using namespace to avoid potential collisions between names and to make your code cleaner. You should never use using namespace std in header files.

(Along the lines of clean code, you should consider using better, longer names for your types; ii, vii, and vvii are atrocious type names).

Answer 2

vector and the like are contained in the namespace std::. Do not use using namespace std; in a header file. Otherwise everyone that includes it gets all of std:: whether intended or not.

On a side note, if this is a utility header intended to be included in other files, you might wrap up those types and #define's in a namespace. Note #define's don't respect namespaces, so you'd prefix them instead:

namespace utility
{
    // ...
    typedef std::queue<int> qi;

    // most would recommend this be in CAPS
    #define utility_tr(c,i) for(typeof((c).begin()) i = (c).begin() ; i!=(c).end() ; ++i )
    // ...    
}

Answer 3

Before your typedefs, you should have using namespace std;

Also, you may wish to use a less common name than UTILS_H.

Answer 4

you should write

using namespace std;

before line

typedef pair<int,int> ii;

or fully qualify types of stl with std::