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

@@ -105,15 +105,24 @@ class code_blockt:public codet
   {
   }
 
-  explicit code_blockt(const std::list<codet> &_list):codet(ID_block)
-  {
-    operandst &o=operands();
-    reserve_operands(_list.size());
-    for(std::list<codet>::const_iterator
-        it=_list.begin();
-        it!=_list.end();
-        it++)
-      o.push_back(*it);
+  static code_blockt from_list(const std::list<codet> &_list)
+  {
+    code_blockt result;
+    operandst &o=result.operands();
+    o.reserve(_list.size());
+    for(const auto &op : _list)
+      o.push_back(op);
+    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)