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);

dpi/cookies.cc

@@ -1329,7 +1329,7 @@ static std::string Cookies_get(char *url_host, char *url_path,
    int i;
 
    if (disabled)
-      return dStrdup("");
+      return "";
 
    matching_cookies = dList_new(8);
 

dpi/datauri.cc

@@ -274,7 +274,7 @@ static unsigned char *datauri_get_data(char *url, size_t *p_sz)
          data = (unsigned char *)a_Url_decode_hex_str(p, p_sz);
       }
    } else {
-      data = (unsigned char *)dStrdup("");
+      data = (unsigned char *)dStrdup("").ptr;
       *p_sz = 0;
    }
 

dw/fltkui.cc

@@ -866,9 +866,9 @@ FltkEntryResource::FltkEntryResource (FltkPlatform *platform, int size,
 {
    this->size = size;
    this->password = password;
-   this->label = label ? dStrdup(label) : NULL;
+   this->label = dStrdup(label);
    this->label_w = 0;
-   this->placeholder = placeholder ? dStrdup(placeholder) : NULL;
+   this->placeholder = dStrdup(placeholder);
 
    initText = NULL;
    editable = false;
@@ -1052,7 +1052,7 @@ FltkMultiLineTextResource::FltkMultiLineTextResource (FltkPlatform *platform,
       MSG_WARN("numRows = %d is set to 1.\n", numRows);
       numRows = 1;
    }
-   this->placeholder = placeholder ? dStrdup(placeholder) : NULL;
+   this->placeholder = dStrdup(placeholder);
 
    init (platform);
 }

dw/hyphenator.cc

@@ -371,7 +371,7 @@ TrieBuilder::~TrieBuilder ()
 void TrieBuilder::insert (const char *key, const char *value)
 {
    dataList->increase ();
-   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key);
+   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key).ptr;
    dataList->getLastRef ()->value = dataZone->strdup (value);
 }
 

lout/object.cc

@@ -290,7 +290,7 @@ void ConstString::intoStringBuffer(misc::StringBuffer *sb) const
 //    String
 // ------------
 
-String::String (const char *str): ConstString (str ? dStrdup(str) : NULL)
+String::String (const char *str): ConstString (dStrdup(str))
 {
 }
 

src/form.cc

@@ -861,7 +861,7 @@ void Html_tag_open_option(DilloHtml *html, const char *tag, int tagsize)
       bool enabled = (a_Html_get_attr(html, tag, tagsize, "disabled") == NULL);
 
       auto option =
-         std::make_unique< DilloHtmlOption >(value ? dStrdup( value.value().c_str() ) : nullptr, label ? dStrdup( label.value().c_str() ) : nullptr, selected, enabled );
+         std::make_unique< DilloHtmlOption >(value ? dStrdup( value.value().c_str() ).ptr : nullptr, label ? dStrdup( label.value().c_str() ).ptr : nullptr, selected, enabled );
 
       input->select->addOpt( std::move( option ) );
    }