Why is undefined behaviour allowed in C

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Undefined behavior is not allowed, it’s just not caught by the compiler.

The tradeoff here is between the speed and the safety. Many kinds of undefined behavior could be prevented at the expense of a few additional CPU cycles.

For example, you could prevent UB that happens when you read from memory that has been allocated but not initialized by having the compiled code write zeros into it. This, however, costs you a whole additional write into a memory, which is entirely unnecessary.

Similarly, one could prevent reading/writing past the end of an array by checking its bounds inside [] operator. However, this would cost you a few additional CPU cycles on each array access.

C++ designers decided that it is better to have speed and allow potential UB than to force everyone pay for what they do not need. This approach, however, is incompatible with Java’s “write once, run anywhere” requirement, so designers of Java language insisted on fully defined behavior in nearly all situations.

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