Move string_printf to maxbase

Can be used in tests.

maxutils/maxbase/include/maxbase/format.hh

@@ -29,4 +29,14 @@ namespace maxbase
  * @return Value as a human readable size e.g. 5.01MiB
  */
 std::string to_binary_size(int64_t size);
+
+/**
+ * Format parameters to a string. Uses printf-formatting.
+ *
+ * @param format Format string
+ * @param ... Items to convert according to format string
+ * @return The result string
+ */
+std::string string_printf(const char* format, ...) mxb_attribute((format (printf, 1, 2)));
+
 }

maxutils/maxbase/src/format.cc

@@ -15,6 +15,8 @@
 
 #include <cmath>
 #include <cstdio>
+#include <maxbase/assert.h>
+#include <maxbase/log.hh>
 
 namespace
 {
@@ -65,4 +67,33 @@ std::string to_binary_size(int64_t size)
     snprintf(buf, sizeof(buf), "%.2lf%s", num, get_binary_size_suffix(idx));
     return buf;
 }
+
+std::string string_printf(const char* format, ...)
+{
+    /* Use 'vsnprintf' for the formatted printing. It outputs the optimal buffer length - 1. */
+    va_list args;
+    va_start(args, format);
+    int characters = vsnprintf(NULL, 0, format, args);
+    va_end(args);
+    std::string rval;
+    if (characters < 0)
+    {
+        // Encoding (programmer) error.
+        mxb_assert(!true);
+        MXB_ERROR("Could not format the string %s.", format);
+    }
+    else if (characters > 0)
+    {
+        // 'characters' does not include the \0-byte.
+        int total_size = characters + 1;
+        rval.reserve(total_size);
+        rval.resize(characters);    // The final "length" of the string
+        va_start(args, format);
+        // Write directly to the string internal array, avoiding any temporary arrays.
+        vsnprintf(&rval[0], total_size, format, args);
+        va_end(args);
+    }
+    return rval;
+}
+
 }