fix: function returns by const value

src/coders/GLSLExtension.cpp

@@ -42,10 +42,10 @@ const std::string& GLSLExtension::getHeader(const std::string& name) const {
     return found->second;
 }
 
-const std::string GLSLExtension::getDefine(const std::string& name) const {
+const std::string& GLSLExtension::getDefine(const std::string& name) const {
     auto found = defines.find(name);
     if (found == defines.end()) {
-        return "";
+        throw std::runtime_error("name '"+name+"' is not defined");
     }
     return found->second;
 }
@@ -84,7 +84,7 @@ inline void source_line(std::stringstream& ss, uint linenum) {
     ss << "#line " << linenum << "\n";
 }
 
-const std::string GLSLExtension::process(const fs::path& file, const std::string& source) {
+std::string GLSLExtension::process(const fs::path& file, const std::string& source) {
     std::stringstream ss;
     size_t pos = 0;
     uint linenum = 1;

src/coders/GLSLExtension.hpp

@@ -24,12 +24,12 @@ public:
     void addHeader(const std::string& name, std::string source);
 
     const std::string& getHeader(const std::string& name) const;
-    const std::string getDefine(const std::string& name) const;
+    const std::string& getDefine(const std::string& name) const;
 
     bool hasHeader(const std::string& name) const;
     bool hasDefine(const std::string& name) const;
 
-    const std::string process(
+    std::string process(
         const std::filesystem::path& file, 
         const std::string& source
     );

src/coders/xml.cpp

@@ -120,7 +120,7 @@ const std::string& Node::getTag() const {
     return tag;
 }
 
-const xmlattribute Node::attr(const std::string& name) const {
+const xmlattribute& Node::attr(const std::string& name) const {
     auto found = attrs.find(name);
     if (found == attrs.end()) {
         throw std::runtime_error("element <"+tag+" ...> missing attribute "+name);
@@ -128,7 +128,7 @@ const xmlattribute Node::attr(const std::string& name) const {
     return found->second;
 }
 
-const xmlattribute Node::attr(const std::string& name, const std::string& def) const {
+xmlattribute Node::attr(const std::string& name, const std::string& def) const {
     auto found = attrs.find(name);
     if (found == attrs.end()) {
         return Attribute(name, def);

src/coders/xml.hpp

@@ -68,14 +68,14 @@ namespace xml {
         /// @param name attribute name
         /// @throws std::runtime_error if element has no attribute 
         /// @return xmlattribute - {name, value}
-        const xmlattribute attr(const std::string& name) const;
+        const xmlattribute& attr(const std::string& name) const;
         
         /// @brief Get attribute by name
         /// @param name attribute name
         /// @param def default value will be returned wrapped in xmlattribute
         /// if element has no attribute 
         /// @return xmlattribute - {name, value} or {name, def} if not found*/
-        const xmlattribute attr(const std::string& name, const std::string& def) const;
+        xmlattribute attr(const std::string& name, const std::string& def) const;
 
         /// @brief Check if element has attribute
         /// @param name attribute name