std::is_signed<char>::value

Understanding char signedness in C++

This is a deceptively simple but important C++ language-knowledge question that trips up many candidates. It tests whether you understand the subtle gap between the language standard and practical compiler behaviour—a distinction that matters deeply in systems programming and financial software where bit-level correctness is non-negotiable.

The question hinges on a quirk of the C++ standard: char has an implementation-defined signedness. Unlike signed char and unsigned char, which are explicit, the default signedness of char itself is left to the compiler. This means code that assumes a particular signedness can behave differently across platforms or compiler flags, silently corrupting arithmetic, comparisons, and array indexing. Understanding this distinction—and knowing how to check it reliably—is essential for writing portable C++ in domains where precision matters.

• Implementation-defined behaviour in the C++ standard
• Type traits and template introspection (std::is_signed)
• Platform-specific compiler defaults
• Sign extension and implicit conversions