core: add locale aware formatting function to std::string

We had locale aware formatting functions that generated QStrings.
Create an alternative that creates std::string, since we want that
in the core.

This commit is unfortunate for two reasons:
- The function is called "casprintf()" for analogy with the QString
  version. However, the non locale aware function is called
  "format_string_std()" for analogy with "format_string()".
  Ultimately these names should be unified. Probably, once there
  are no membuffer users left.
- This does UTF-16->UTF-8->UTF-16 roundtrips. The core formatting
  functions should render UTF-8 and only convert to UTF-16, in
  the UI layer.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/format.cpp

@@ -407,3 +407,13 @@ extern "C" void put_vformat_loc(struct membuffer *b, const char *fmt, va_list ar
 	memcpy(b->buffer + b->len, data, utf8_size);
 	b->len += utf8_size;
 }
+
+// TODO: Avoid back-and-forth conversion between UTF16 and UTF8.
+std::string casprintf_loc(const char *cformat, ...)
+{
+	va_list ap;
+	va_start(ap, cformat);
+	QByteArray utf8 = vqasprintf_loc(cformat, ap).toUtf8();
+	va_end(ap);
+	return std::string(utf8.constData(), utf8.size());
+}

core/format.h

@@ -11,6 +11,7 @@
 #include <QString>
 __printf(1, 2) QString qasprintf_loc(const char *cformat, ...);
 __printf(1, 0) QString vqasprintf_loc(const char *cformat, va_list ap);
+__printf(1, 2) std::string casprintf_loc(const char *cformat, ...);
 #endif
 
 #ifdef __cplusplus