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To check if a specific identifier is defined in C++, you can use the preprocessor directive #ifdef. This directive checks if the given identifier has been defined using a #define statement earlier in the code. The syntax is as follows:

#ifdef identifier
   // code to be executed if the identifier is defined
#else
   // code to be executed if the identifier is not defined
#endif

Here's a step-by-step breakdown of what each part of the code does:

1. #ifdef identifier: This line checks if the identifier is defined. If it is defined, the code following this line is executed. If it is not defined, the code following the #else directive (if present) is executed.
2. // code to be executed if the identifier is defined: This is the code block that will be executed if the identifier is defined. You can place any valid C++ code within this block.
3. #else: This optional line is used to specify an alternative code block that will be executed if the identifier is not defined.
4. // code to be executed if the identifier is not defined: This is the code block that will be executed if the identifier is not defined. Again, you can place any valid C++ code within this block.
5. #endif: This line marks the end of the conditional compilation block.

By using the #ifdef directive, you can conditionally execute different parts of your code based on whether a specific identifier is defined or not.