Understanding C++ Value Categories
This article aims to explain and clarify the different categories of values in C++. Historically, only lvalues and rvalues were used to classify expressions, but with the evolution of the language especially since C++11, new categories have been introduced to capture more precise semantics.
Value categories are one of the most fundamental yet misunderstood concepts in modern C++. Understanding them is crucial for writing efficient code, especially when working with move semantics and perfect forwarding.
Quick Reference Guide
Variable name
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Function returning T&
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Function returning T&&
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Function returning T
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std::move(x)
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Literal (42, 3.14)
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String literal "hello"
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x + y
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++x
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x++
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*ptr
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Reminder:
lvalues have identity and persist beyond expressions
prvalues are temporary values without identity
xvalues have identity but are candidates for moving
glvalues (lvalue + xvalue) have identity
rvalues (xvalue + prvalue) can be moved from
Conclusion
Understanding C++ value categories is essential for modern C++ development. They're the foundation for move semantics, perfect forwarding, and optimization. While the terminology can seem daunting at first, these concepts enable you to write more efficient, expressive code.
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