diff --git a/include/stdex/string.hpp b/include/stdex/string.hpp
index ae4831c41..4c0e03735 100644
--- a/include/stdex/string.hpp
+++ b/include/stdex/string.hpp
@@ -804,4 +804,97 @@ namespace stdex
 		return (size_t)strtou32(str, count, end, radix);
 #endif
 	}
+
+	/// \cond internal
+	inline int vsnprintf(_Out_z_cap_(capacity) char *str, _In_ size_t capacity, _In_z_ _Printf_format_string_ const char *format, _In_ va_list arg)
+	{
+#if _MSC_VER <= 1600
+#pragma warning(suppress: 4996)
+		return _vsnprintf(str, capacity, format, arg);
+#else
+#pragma warning(suppress: 4996)
+		return ::vsnprintf(str, capacity, format, arg);
+#endif
+	}
+
+	inline int vsnprintf(_Out_z_cap_(capacity) wchar_t *str, _In_ size_t capacity, _In_z_ _Printf_format_string_ const wchar_t *format, _In_ va_list arg) noexcept
+	{
+#pragma warning(suppress: 4996)
+		return _vsnwprintf(str, capacity, format, arg);
+	}
+	/// \endcond
+
+	///
+	/// Formats string using `printf()`.
+	///
+	/// \param[out] str     String to append formatted text
+	/// \param[in ] format  String template using `printf()` style
+	/// \param[in ] arg     Arguments to `format`
+	///
+	template<class _Elem, class _Traits, class _Ax>
+	inline void vappendf(_Inout_ std::basic_string<_Elem, _Traits, _Ax> &str, _In_z_ _Printf_format_string_ const _Elem *format, _In_ va_list arg)
+	{
+		_Elem buf[1024/sizeof(_Elem)];
+
+		// Try with stack buffer first.
+		int count = vsnprintf(buf, _countof(buf) - 1, format, arg);
+		if (count >= 0) {
+			// Copy from stack.
+			str.append(buf, count);
+		} else {
+			for (size_t capacity = 2*1024/sizeof(_Elem);; capacity *= 2) {
+				// Allocate on heap and retry.
+				auto buf_dyn = std::make_unique<_Elem[]>(capacity);
+				count = vsnprintf(buf_dyn.get(), capacity - 1, format, arg);
+				if (count >= 0) {
+					str.append(buf_dyn.get(), count);
+					break;
+				}
+			}
+		}
+	}
+
+	///
+	/// Formats string using `printf()`.
+	///
+	/// \param[out] str     String to append formatted text
+	/// \param[in ] format  String template using `printf()` style
+	///
+	template<class _Elem, class _Traits, class _Ax>
+	inline void appendf(_Inout_ std::basic_string<_Elem, _Traits, _Ax> &str, _In_z_ _Printf_format_string_ const _Elem *format, ...)
+	{
+		va_list arg;
+		va_start(arg, format);
+		vappendf(str, format, arg);
+		va_end(arg);
+	}
+
+	///
+	/// Formats string using `printf()`.
+	///
+	/// \param[out] str     Formatted string
+	/// \param[in ] format  String template using `printf()` style
+	/// \param[in ] arg     Arguments to `format`
+	///
+	template<class _Elem, class _Traits, class _Ax>
+	inline void vsprintf(_Inout_ std::basic_string<_Elem, _Traits, _Ax> &str, _In_z_ _Printf_format_string_ const _Elem *format, _In_ va_list arg)
+	{
+		str.clear();
+		appendf(str, format, arg);
+	}
+
+	///
+	/// Formats string using `printf()`.
+	///
+	/// \param[out] str     Formatted string
+	/// \param[in ] format  String template using `printf()` style
+	///
+	template<class _Elem, class _Traits, class _Ax>
+	inline void sprintf(_Inout_ std::basic_string<_Elem, _Traits, _Ax> &str, _In_z_ _Printf_format_string_ const _Elem *format, ...)
+	{
+		va_list arg;
+		va_start(arg, format);
+		vsprintf(str, format, arg);
+		va_end(arg);
+	}
 }
\ No newline at end of file