new constructors for code_blockt

The new constructors avoid building a vector from a list.  The vector is
constructed directly at the call site of the constructor.

Author: Daniel Kroening

src/ansi-c/c_typecheck_code.cpp

@@ -353,7 +353,7 @@ void c_typecheck_baset::typecheck_decl(codet &code)
   else
   {
     // build a decl-block
-    code_blockt code_block(new_code);
+    auto code_block=code_blockt::from_list(new_code);
     code_block.set_statement(ID_decl_block);
     code.swap(code_block);
   }

src/ansi-c/c_typecheck_expr.cpp

@@ -1010,7 +1010,7 @@ void c_typecheck_baset::typecheck_expr_sizeof(exprt &expr)
   {
     side_effect_exprt side_effect_expr(
       ID_statement_expression, void_type(), expr.source_location());
-    code_blockt decl_block(clean_code);
+    auto decl_block=code_blockt::from_list(clean_code);
     decl_block.set_statement(ID_decl_block);
     side_effect_expr.copy_to_operands(decl_block);
     clean_code.clear();
@@ -1067,7 +1067,7 @@ void c_typecheck_baset::typecheck_expr_typecast(exprt &expr)
   {
     side_effect_exprt side_effect_expr(
       ID_statement_expression, void_type(), expr.source_location());
-    code_blockt decl_block(clean_code);
+    auto decl_block=code_blockt::from_list(clean_code);
     decl_block.set_statement(ID_decl_block);
     side_effect_expr.copy_to_operands(decl_block);
     clean_code.clear();

src/util/std_code.h

@@ -117,15 +117,24 @@ class code_blockt:public codet
     return (const code_operandst &)get_sub();
   }
 
-  explicit code_blockt(const std::list<codet> &_list):codet(ID_block)
+  static code_blockt from_list(const std::list<codet> &_list)
   {
-    auto &o = statements();
-    reserve_operands(_list.size());
-    for(std::list<codet>::const_iterator
-        it=_list.begin();
-        it!=_list.end();
-        it++)
-      o.push_back(*it);
+    code_blockt result;
+    statementst &s=result.statements();
+    s.reserve(_list.size());
+    for(const auto &c : _list)
+      o.push_back(c);
+    return result;
+  }
+
+  explicit code_blockt(const std::vector<codet> &_statements):codet(ID_block)
+  {
+    operands()=(const std::vector<exprt> &)_statements;
+  }
+
+  explicit code_blockt(std::vector<codet> &&_statements):codet(ID_block)
+  {
+    operands()=std::move((std::vector<exprt> &&)_statements);
   }
 
   void move(codet &code)
@@ -166,9 +175,9 @@ class code_blockt:public codet
       const irep_idt &statement=last->get_statement();
 
       if(statement==ID_block &&
-         !last->operands().empty())
+         !to_code_block(*last).statements().empty())
       {
-        last=&to_code(last->operands().back());
+        last=&to_code_block(*last).statements().back());
       }
       else if(statement==ID_label)
       {