avoid designing by coding

I found a obvious, stupid mistake in my c++ code. The mistake itself is trivial and doesn't worth mentioning. What I'm interested in is what lead me to making such a mistake that a very fresh c++ learner can easily recognize.
The main reason is I was designing by coding (implementing). Designing and coding are very separate tasks in a programming project. The goal of designing is to find out an effective way of organizing our code (classes, modules, layers, etc.) so that the code is easy to understand, maintain and to be changed. The goal of coding is implementing required features based on design. Because designing and coding have different goals, there are different ways to do them. But I chose to do them simultaneously, that is, thought about the design and wrote code immediately, even before I had a clear idea of the whole design. In this way, I always didn't have a sole goal in mind, but had to think about multiple things. It's not natural for human brain to work this way. Especially considering the complexity of c++ language, if we can't fully focus on the implementation details, we are very likely to be bitten by it. The case might be less painful if we work with other languages like c#, python, which have much less details to concern. But it's generally wise to choose a more suitable tool to do the design other than coding, for example UML, or other diagrams. And code review is a good way to get rid of such mistakes.