Extra Clang Tools 3.3 documentation

Pass-By-Value Transform

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Pass-By-Value Transform

The Pass-By-Value Transform makes use of the pass-by-value idiom when possible.

With move semantics added to the language and the standard library updated with move constructors added for many types it is now interesting to take an argument directly by value, instead of by const-reference, and then copy. This transformation allows the compiler to take care of choosing the best way to construct the copy.

The transformation is usually beneficial when the calling code passes an rvalue and assumes the move construction is a cheap operation. This short example illustrates how the construction of the value happens:

void foo(std::string s);
std::string get_str();

void f(const std::string &str) {
  foo(str);       // lvalue  -> copy construction
  foo(get_str()); // prvalue -> move construction
}

Note

Currently only constructors are transformed to make use of pass-by-value. Contributions that handle other situations are welcome!

Pass-by-value in constructors

Replaces the uses of const-references constructor parameters that are copied into class fields. The parameter is then moved with std::move().

Since std::move() is a library function declared in <utility> it may be necessary to add this include. The transform will add the include directive when necessary.

Example:

$ clang-modernize -pass-by-value ctor.cpp

ctor.cpp

 #include <string>

 class Foo {
 public:
-  Foo(const std::string &Copied, const std::string &ReadOnly)
-    : Copied(Copied), ReadOnly(ReadOnly)
+  Foo(std::string Copied, const std::string &ReadOnly)
+    : Copied(std::move(Copied)), ReadOnly(ReadOnly)
   {}

 private:
   std::string Copied;
   const std::string &ReadOnly;
 };

 std::string get_cwd();

 void f(const std::string &Path) {
   // The parameter corresponding to 'get_cwd()' is move-constructed. By
   // using pass-by-value in the Foo constructor we managed to avoid a
   // copy-construction.
   Foo foo(get_cwd(), Path);
 }

If the parameter is used more than once no transformation is performed since moved objects have an undefined state. It means the following code will be left untouched:

#include <string>

void pass(const std::string &S);

struct Foo {
  Foo(const std::string &S) : Str(S) {
    pass(S);
  }

  std::string Str;
};

Risk

This modification is considered reasonably safe (see -risk option).

A situation where the generated code can be wrong is when the object referenced is modified before the assignment in the init-list through a “hidden” reference.

Example:

 std::string s("foo");

 struct Base {
   Base() {
     s = "bar";
   }
 };

 struct Derived : Base {
-  Derived(const std::string &S) : Field(S)
+  Derived(std::string S) : Field(std::move(S))
   { }

   std::string Field;
 };

 void f() {
-  Derived d(s); // d.Field holds "bar"
+  Derived d(s); // d.Field holds "foo"
 }

Note about delayed template parsing

When delayed template parsing is enabled, constructors part of templated contexts; templated constructors, constructors in class templates, constructors of inner classes of template classes, etc., are not transformed. Delayed template parsing is enabled by default on Windows as a Microsoft extension: Clang Compiler User’s Manual - Microsoft extensions.

Delayed template parsing can be enabled using the -fdelayed-template-parsing flag and disabled using -fno-delayed-template-parsing.

Example:

  template <typename T> class C {
    std::string S;

  public:
=  // using -fdelayed-template-parsing (default on Windows)
=  C(const std::string &S) : S(S) {}

+  // using -fno-delayed-template-parsing (default on non-Windows systems)
+  C(std::string S) : S(std::move(S)) {}
  };

See also

For more information about the pass-by-value idiom, read: Want Speed? Pass by Value.

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