diff --git a/lib/rscons/basic_environment.rb b/lib/rscons/basic_environment.rb
index f3642f4..2e18134 100644
--- a/lib/rscons/basic_environment.rb
+++ b/lib/rscons/basic_environment.rb
@@ -13,18 +13,42 @@ module Rscons
       load_configuration_data!(options)
     end
 
-    # Get a construction variable's value.
+    # Access the value of a construction variable.
     #
-    # @see VarSet#[]
-    def [](*args)
-      @varset.__send__(:[], *args)
+    # @param key [String, Symbol]
+    #   The construction variable name.
+    #
+    # @return [Object]
+    #   The construction variable's value.
+    def [](key)
+      @varset[key]
     end
 
-    # Set a construction variable's value.
+    # Access the value of a construction variable.
     #
-    # @see VarSet#[]=
-    def []=(*args)
-      @varset.__send__(:[]=, *args)
+    # This method is similar to #[] but does not make a copy-on-access copy of
+    # the variable accessed. This means that the returned value is NOT safe to
+    # be modified by the caller. Thus the caller must guarantee that it does
+    # not modify the returned value.
+    #
+    # @param key [String, Symbol]
+    #   The construction variable name.
+    #
+    # @return [Object]
+    #   The construction variable's value.
+    def get_var(key)
+      @varset.get_var(key)
+    end
+
+    # Assign a value to a construction variable.
+    #
+    # @param key [String, Symbol]
+    #   The construction variable name.
+    #
+    # @param val [Object]
+    #   The value to set.
+    def []=(key, val)
+      @varset[key] = val
     end
 
     # Add a set of construction variables to the BasicEnvironment.
@@ -108,7 +132,7 @@ module Rscons
             append["LDFLAGS", ["-arch", val]]
           end
           skip = true
-        elsif word =~ /^#{self["CPPDEFPREFIX"]}(.*)$/
+        elsif word =~ /^#{get_var("CPPDEFPREFIX")}(.*)$/
           handle["CPPDEFINES", $1]
         elsif word == "-include"
           if val = words[i + 1]
@@ -121,11 +145,11 @@ module Rscons
             append["LDFLAGS", ["-isysroot", val]]
           end
           skip = true
-        elsif word =~ /^#{self["INCPREFIX"]}(.*)$/
+        elsif word =~ /^#{get_var("INCPREFIX")}(.*)$/
           handle["CPPPATH", $1]
-        elsif word =~ /^#{self["LIBLINKPREFIX"]}(.*)$/
+        elsif word =~ /^#{get_var("LIBLINKPREFIX")}(.*)$/
           handle["LIBS", $1]
-        elsif word =~ /^#{self["LIBDIRPREFIX"]}(.*)$/
+        elsif word =~ /^#{get_var("LIBDIRPREFIX")}(.*)$/
           handle["LIBPATH", $1]
         elsif word == "-mno-cygwin"
           append["CCFLAGS", [word]]
@@ -174,7 +198,7 @@ module Rscons
     # @return [void]
     def merge_flags(flags)
       flags.each_pair do |key, val|
-        if self[key].is_a?(Array) and val.is_a?(Array)
+        if self.get_var(key).is_a?(Array) and val.is_a?(Array)
           self[key] += val
         else
           self[key] = val
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 9124653..49fc1bb 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -507,9 +507,9 @@ module Rscons
     # @return [String] The command's standard output.
     def shell(command)
       shell_cmd =
-        if self["SHELL"]
-          flag = self["SHELLFLAG"] || (self["SHELL"] == "cmd" ? "/c" : "-c")
-          [self["SHELL"], flag]
+        if shell = get_var("SHELL")
+          flag = get_var("SHELLFLAG") || (shell == "cmd" ? "/c" : "-c")
+          [shell, flag]
         else
           Rscons.get_system_shell
         end
diff --git a/lib/rscons/varset.rb b/lib/rscons/varset.rb
index 2199d9f..f6b6161 100644
--- a/lib/rscons/varset.rb
+++ b/lib/rscons/varset.rb
@@ -11,11 +11,13 @@ module Rscons
       append(vars)
     end
 
-    # Access the value of variable.
+    # Access the value of a variable.
     #
-    # @param key [String, Symbol] The variable name.
+    # @param key [String, Symbol]
+    #   The variable name.
     #
-    # @return [Object] The variable's value.
+    # @return [Object]
+    #   The variable's value.
     def [](key)
       if @my_vars.include?(key)
         @my_vars[key]
@@ -30,11 +32,38 @@ module Rscons
       end
     end
 
+    # Access the value of a variable.
+    #
+    # This method is similar to #[] but does not make a copy-on-access copy of
+    # the variable accessed. This means that the returned value is NOT safe to
+    # be modified by the caller. Thus the caller must guarantee that it does
+    # not modify the returned value.
+    #
+    # @param key [String, Symbol]
+    #   The variable name.
+    #
+    # @return [Object]
+    #   The variable's value.
+    def get_var(key)
+      if @my_vars.include?(key)
+        @my_vars[key]
+      else
+        @coa_vars.each do |coa_vars|
+          if coa_vars.include?(key)
+            return coa_vars[key]
+          end
+        end
+        nil
+      end
+    end
+
     # Assign a value to a variable.
     #
-    # @param key [String, Symbol] The variable name.
+    # @param key [String, Symbol]
+    #   The variable name.
     #
-    # @param val [Object] The value to set.
+    # @param val [Object]
+    #   The value to set.
     def []=(key, val)
       @my_vars[key] = val
     end
@@ -99,7 +128,7 @@ module Rscons
         if varref =~ /^(.*)\$\{([^}]+)\}(.*)$/
           prefix, varname, suffix = $1, $2, $3
           prefix = expand_varref(prefix, lambda_args) unless prefix.empty?
-          varval = expand_varref(self[varname], lambda_args)
+          varval = expand_varref(get_var(varname), lambda_args)
           # suffix needs no expansion since the regex matches the last occurence
           case varval
           when Array
@@ -115,7 +144,7 @@ module Rscons
               "#{prefix}#{varval}#{suffix}"
             end
           else
-            raise "Unknown construction variable type: #{varval.class} (from #{varname.inspect} => #{self[varname].inspect})"
+            raise "Unknown construction variable type: #{varval.class} (from #{varname.inspect} => #{get_var(varname).inspect})"
           end
         else
           varref
diff --git a/spec/rscons/varset_spec.rb b/spec/rscons/varset_spec.rb
index ba920ef..14ffc5d 100644
--- a/spec/rscons/varset_spec.rb
+++ b/spec/rscons/varset_spec.rb
@@ -169,7 +169,7 @@ module Rscons
         expect { v.expand_varref({a: :b}, :lambda_args) }.to raise_error /Unknown construction variable type: Hash/
       end
       it "raises an error when an expanded variable is an unexpected type" do
-        expect(v).to receive(:[]).at_least(1).times.with("bad").and_return("bad_val")
+        expect(v).to receive(:get_var).at_least(1).times.with("bad").and_return("bad_val")
         expect(v).to receive(:expand_varref).with("bad_val", :lambda_args).and_return({a: :b})
         expect(v).to receive(:expand_varref).and_call_original
         expect { v.expand_varref("${bad}", :lambda_args) }.to raise_error /Unknown construction variable type: Hash/