std::setw() and Stringstreams: Sticky vs. Non-Sticky Manipulators—What's the Difference?

Manipulators vs. Format Objects

While attempting to construct a stringstream, a common misconception arises regarding the behavior of std::setw(). Assuming it would perpetually affect the stringstream was an incorrect assumption, as it is reset after each insertion. This article aims to clarify the distinction between sticky and non-sticky manipulators and format objects, addressing the following questions:

• Why is std::setw() behaving in this manner?
• Are there other manipulators with similar behavior?
• How do std::ios_base::width() and std::setw() differ in terms of behavior?
• Where can clear documentation regarding this behavior be found online?

Sticky Manipulators

The manipulators within std::ios_base can be classified into two categories:

• Manipulators returning objects: These manipulators do not directly affect the stream object and thus do not exhibit "stickiness." This includes setw.
• Manipulators returning streams: These manipulators directly impact the stream object and are thus "sticky," meaning the modifications they make persist until explicitly changed by another manipulator. Examples include setiosflags and setfill.

Format Objects

Format objects are employed to temporally alter the formatting properties of a stream. Contrary to manipulators, they do not affect the stream object directly. An example of this is the PutSquareBracket format object, which allows for temporary modification of formatting without affecting the permanent state.

Conclusion

std::setw() is unique among manipulators in that it is the only non-sticky one. The lack of documentation regarding its specific behavior may be attributed to its perceived similarity to other manipulators. However, the reality is that it behaves differently due to the requirement in formatted output operations to explicitly set the width to zero.

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