add doc: substr

source/libs/string/basic_string/substr.rst

@@ -0,0 +1,44 @@
+.. _sprout-string-basic_string-substr:
+###############################################################################
+substr
+###############################################################################
+
+Interface
+========================================
+.. sourcecode:: c++
+
+  SPROUT_CONSTEXPR basic_string substr(size_type pos = 0, size_type n = npos) const;
+
+Requires
+========================================
+
+| ``pos <= size()``.
+
+Returns
+========================================
+
+| ``basic_string(data() + pos, rlen)`` when rlen is the smaller of n and ``size() - pos``.
+
+Throws
+========================================
+
+| std::out_of_range if ``pos > size()``.
+
+Examples
+========================================
+.. sourcecode:: c++
+
+  #include <sprout/string.hpp>
+  #include <sprout/assert.hpp>
+  using namespace sprout;
+  
+  SPROUT_STATIC_CONSTEXPR auto x = string<8>("homuhomu");
+  SPROUT_STATIC_CONSTEXPR auto y = x.substr(4, 4);
+  SPROUT_ASSERT_MASG(y == "homu", "y is substring of x.");
+
+Header
+========================================
+
+| ``sprout/string/string.hpp``
+| Convenience header: ``sprout/string.hpp``
+