60 Why Not to Use using namespace std

Why I personally don't use using namespace std, this is just an opinion.

1. What is using namespace

As in the code above, which uses a lot of standard library content, if you add the following at the beginning of the code:

using namespace std;

You can directly write vector, find_if, etc., making the code look cleaner.
You can also limit it to a scope, for example, writing it at the beginning of the main function, so that calls to the standard library within the main function don't need to be prefixed with std::.

So using namespace can be very useful, especially if you are dealing with long namespaces, or if you have your own namespace where all the symbols in your project files are within this namespace, you can use this.

But I personally don't like using namespace std.

2. Why I don't like it

At first glance, the code looks cleaner, but if you look at the original code, you can easily identify the use of the C++ Standard Template Library (with the std prefix). If using namespace std is used, it becomes relatively harder to distinguish. If you also use the snake case naming convention (like find_if) that the standard library prefers, it becomes difficult to tell whether it's from std or not.

Let me show you a real-life example. Cherno used to work in the Frostbite Engine department at EA, where they used EASTL (a replacement for C++ STL), which uses the eastl namespace. If you just write vector, how do I know if it's eastl::vector or std::vector?

namespace apple{

    void print(const std::string& text)
    {
        std::cout << text << std::endl;
    }
}

using namespace apple;
int main()
{
    // apple::print("Hello");
    print("Hello");

Now there's a new namespace orange, which also has a print function, but it prints the reversed content.

namespace orange{
    void print(const char* text)
    {
        std::string temp = text;
        std::reverse(temp.begin(),temp.end());
        std::cout << temp << std::endl;
    }
}

using namespace apple;
using namespace orange;

int main()
{
    print("Hello");

So which one will be called here?

This is not because orange is after apple, but for another reason. "Hello" is actually a const char[] array, not a string. If only the apple namespace is used, an implicit conversion will occur in apple::print(), converting the const char array to a string object. But after introducing the orange namespace, orange::print() has a better match because its parameter is exactly a const char*, requiring no implicit conversion.

If we don't use using namespace, but simply include another library apple::print(), such runtime errors can be avoided.

Another thing to absolutely avoid is using using namespace in header files. Never do this, as it applies these namespaces to places you didn't intend to use them. Who knows what it might include? It's very difficult to track in any large project, so never use using namespace in header files!