Why Doesn't My C Code Always Need `#include `?

Omission of "#include " in C Compilation

Despite the requirement to include the necessary headers in C code, there are instances where the omission of the "#include " header may not always lead to compilation failures. However, relying on this behavior is unreliable and can result in unexpected issues.

If code snippets utilize members defined within the "string" header, its inclusion is mandatory, either directly or indirectly via other header files. While some compilers on specific platforms may occasionally compile code without the required header, this behavior is unpredictable and not recommended as a practice.

The apparent resolution of such code without explicit inclusion of "#include " stems from the fact that other standard headers included in the code may also include the "string" header. However, this dependency is unreliable and can vary based on the compiler version and configuration.

To ensure reliable compilation, it's crucial to include all necessary headers explicitly. Unfortunately, comprehensive online documentation on required headers is not readily available. Instead, refer to established C books or the official C standard for guidance.

For instance, certain compilers may compile the following code without "#include ":

#include <iostream>

int main() {
    std::string str;
}

However, removing the first line results in a compilation error, demonstrating the unreliable nature of relying on implicit header inclusion.

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