How to get the length of a string in C ? (.length() vs .size())

在C 中，std::string的.length()和.size()功能完全相同，均返回字符数量（size_t类型）；.length()侧重文本语义，.size()契合STL容器统一接口；二者对空串、含空格及'\0'的字符串均准确计数。

In C , you can get the length of a std::string using either .length() or .size() — they do exactly the same thing and return the number of characters as a size_t.

What’s the difference between .length() and .size()?

There is no practical difference. Both are member functions of std::string, and both return identical values. The choice is mostly stylistic or contextual:

• .length() emphasizes the idea of “how many characters are in this string” — more natural when thinking about text.
• .size() aligns with the standard container interface (e.g., std::vector::size(), std::array::size()) — preferred if you're treating strings uniformly with other STL containers.

How to use them safely

Both return size_t, which is an unsigned type. Be careful when mixing with signed integers or doing arithmetic that could underflow:

• Avoid comparing str.length() - 1 — it will always be false since <code>size_t wraps around on underflow.
• Cast to a signed type only when necessary, and verify the string isn’t empty first.
• Prefer !str.empty() over str.length() > 0 for readability and efficiency.

What about C-style strings?

If you’re working with null-terminated C-style strings (char*), neither .length() nor .size() applies. Use std::strlen() instead:

Empty and edge cases

Both methods work consistently for all valid std::string states:

• std::string s; → s.length() == 0 and s.size() == 0
• std::string s = ""; → same result
• std::string s = "a"; → both return 1
• They count all characters, including spaces, tabs, and null bytes ('\0') — unlike C-style string functions.

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