String alloc shouldn't convert to std::string.

As I work through making code use more C++ RAII and such, most
of the work is handling strings, especially temporaries.  As member
variables which manage string memory get turned into `std::string`,
some use cases might wind up leaking memory.  (One was found in
this change.)

By using a non-convertible-to-string result, such accidents should
be avoided.

dlib/dlib.cc

@@ -74,32 +74,32 @@ void dFree (void *mem)
  *- strings (char *) ----------------------------------------------------------
  */
 
-char *dStrdup(const char *s)
+CharPtrNoStringConversion dStrdup(const char *s)
 {
    if (s) {
       int len = strlen(s)+1;
       char *ns = dNew(char, len);
       memcpy(ns, s, len);
-      return ns;
+      return { ns };
    }
-   return NULL;
+   return {};
 }
 
-char *dStrndup(const char *s, size_t sz)
+CharPtrNoStringConversion dStrndup(const char *s, size_t sz)
 {
    if (s) {
       char *ns = dNew(char, sz+1);
       memcpy(ns, s, sz);
       ns[sz] = 0;
-      return ns;
+      return { ns };
    }
-   return NULL;
+   return {};
 }
 
 /**
  * Concatenate a NULL-terminated list of strings
  */
-char *dStrconcat(const char *s1, ...)
+CharPtrNoStringConversion dStrconcat(const char *s1, ...)
 {
    va_list args;
    char *s, *ns = NULL;
@@ -113,7 +113,7 @@ char *dStrconcat(const char *s1, ...)
       ns = dstr->str;
       dStr_free(dstr, 0);
    }
-   return ns;
+   return { ns };
 }
 
 /**
@@ -146,11 +146,11 @@ void dStrshred(char *s)
 /**
  * Return a new string of length 'len' filled with 'c' characters
  */
-char *dStrnfill(size_t len, char c)
+CharPtrNoStringConversion dStrnfill(size_t len, char c)
 {
    char *ret = dNew(char, len+1);
    for (ret[len] = 0; len > 0; ret[--len] = c);
-   return ret;
+   return { ret };
 }
 
 /**

dlib/dlib.hh

@@ -11,6 +11,13 @@
 
 #include "d_size.h"
 
+struct CharPtrNoStringConversion
+{
+	char *ptr= nullptr;
+
+	operator char *() const noexcept { return ptr; }
+};
+
 #ifdef __cplusplus
 extern "C" {
 #endif /* __cplusplus */
@@ -84,11 +91,11 @@ void dFree (void *mem);
 /*
  *- C strings -----------------------------------------------------------------
  */
-char *dStrdup(const char *s);
-char *dStrndup(const char *s, size_t sz);
-char *dStrconcat(const char *s1, ...);
+CharPtrNoStringConversion dStrdup(const char *s);
+CharPtrNoStringConversion dStrndup(const char *s, size_t sz);
+CharPtrNoStringConversion dStrconcat(const char *s1, ...);
 char *dStrstrip(char *s);
-char *dStrnfill(size_t len, char c);
+CharPtrNoStringConversion dStrnfill(size_t len, char c);
 char *dStrsep(char **orig, const char *delim);
 void dStrshred(char *s);
 char *dStriAsciiStr(const char *haystack, const char *needle);