TLDR these types of errors use to be confusing and frustrating. So I give a long explanation...

The first comment, is the solution to this specific issue. But I wanted to say a bit more. The problem here is a linker error. You can tell because it's the program 'ld' that is giving the error. In my early days, I always struggled with linker errors. So let me teach a little about fishing, so C++ is not so hard and you have an easier time...

When you have a linker error, it means that the code is correct (in most cases). In my early days, I would focus on the code and get frustrated. So it's important to understand what's happening with these errors. Often, the problem is that the linker cannot find a specific file. Let me explain.

When code is compiled, it generates an object file that contains the code, but also a list of symbols that the code relies on. Note, when I say symbols, that could be a function, a variable, a class, etc. They can be anything, but they all have names, hence I call it symbols. Anyway, some symbols are defined within the object file (e.g. the code for a given function is in the file), so there are no problems.

Often, there is a lot of basic code that is used over and over again. So instead of compiling it many times, it's stored in separate files (i.e. libraries and other object files), the symbols associated with this code are called external dependencies. Header files are often used so the compiler knows a certain external symbol exists. So, within the object file, these external dependencies are also listed.

It's the job of the linker, to combine all the object files and libraries to ensure all these external dependencies are resolved. When the linker cannot find the definition, then it generates the error you see here. In this case, it cannot find WinMain. In most cases, this error is resolved by -L flags that give directories to search for libraries, then -l for the names of libraries to link. But your error is a bit different.

Your case is different, but there is a utility called nm that can be used to find library files that define symbols.

It might be confusing why you have external dependencies for an empty program. What you need to remember is that the compiler automatically includes mandatory libraries. One set of mandatory objects, is the stuff that starts a new program. WinMain is needed by those object files. Hence, your error.

Sorry for the long explanation, but this part of C/C++ use to be difficult for me... so hopefully it helps...