c++ - 'list' was not declared in this scope

Wednesday, August 21, 2019

c++ - 'list' was not declared in this scope

Answer

I am new to c++, and I am trying to get a basic program to initialize a list of short unsigned integers. I am compiling and running using scygwin and g++.

Below is the code in the .cpp file:

#include 
#include 
#include 
#include 
#include 
using namespace std;


int main (int argc, char* argv[]) {


list events;

return 0;
}

which I run by typing the following command into cygwin terminal:

$ g++ -o test.out test.cpp

However, I get the following compilation errors:

test.cpp: In function ‘int main(int, char**)’: test.cpp:16:1: error:
‘list’ was not declared in this scope list events;
^
test.cpp:16:6: error: expected primary-expression before ‘int’
list events;
^

I am confused about why list is not in the scope, since I am using namespace std? I found a similar question asked about this on a c++ forum, but my problem would be resolved with that. Anyone know what the problem is here?

-Paul

Answer

using namespace std; doesn't add any functionality to your code. It just means you don't have to type std:: when referencing things in the std namespace, like std::list.

To actually include the code base for std::list into your program, you need to add:

#include

When in doubt about this kind of thing, doing a google search for cpp reference list will turn up a page like this where you can see: Defined in header at the top.

Here's another question about using namespace std; that may prove useful and why you shouldn't use it. I'll add a little bit to perhaps explain namespaces.

It is common in C++ programs to organize functions into classes and namespaces. Imagine you wrote your own list class to handle certain scenarios. In order to prevent naming conflicts you would put it in a different namespace than std.

namespace MyApp {

    class list;
    void sort(list&);
}

For the majority of a large code base you might still prefer to use std::list but you need MyApp::list for some things. Using namespaces you can cluster your code and prevent naming conflicts for similar functionality.

Summary

using namespace std; makes it so that if you reference a function or class not in the global namespace it looks for it in the std namespace.

#include actually inserts prototypes (information about how to access the code) in your source file during the preprocessor stage.

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Wednesday, August 21, 2019

c++ - 'list' was not declared in this scope

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