Refactor: Migrate all prefs to string_view
There should be no behavioral changes, just how the parameters are
taken.
What's wrong with std::string:
1. Data is heap allocated. Most usecase of pref involves look up
some value with a compile time constant string. For example,
|prefs.GetKey(kPrefsManifestBytes)|. When this code is executed,
what it's doing is create a std::instance with kPrefsManifestBytes,
which is a const char *. The program must first determine the length
of |kPrefsManifestBytes|, allocate sufficient heap memory to store
it, then copy all contents over. After prefs.GetKey() is called, the
allocated string must be deallocated. Everytime we call GetKey()
with a const char *, we execute the same
strlen()-allocate()-copy()-deallocate() sequence. Which is not
efficient.
2. Often requires passing by reference, which introduces 1 more
pointer indirection
Why/How std::string_view fixes these problems
1. std::string_view does not own the underlying data. When you do
std::string_view{kPrefsManifestBytes}, it merely takes the pointer
in, store it, then compute the size. No heap allocation happens.
However, since std::string_view doesn't own the underlying data,
lifetime of std::string_view cannot exceed that of const char *.
This is fine for our usecase, because constants like
kPrefsManifestBytes are static constants and are valid thorugh out
the entire program execution
2. Since std::string_view is virtuall a tuple<const char *data,
size_t size>, no need to pass it by reference. It can be efficiently
passed around by value, reducing pointer indirection.
Side note:
std::string_view is essentially the C++ equivalence of go/java-tips/012
Test: th
Change-Id: I4c3f1d88a0587e36ac5eca43d553448da1e2e878
diff --git a/common/fake_prefs.h b/common/fake_prefs.h
index 430c291..7ae9fb9 100644
--- a/common/fake_prefs.h
+++ b/common/fake_prefs.h
@@ -17,6 +17,7 @@
#ifndef UPDATE_ENGINE_COMMON_FAKE_PREFS_H_
#define UPDATE_ENGINE_COMMON_FAKE_PREFS_H_
+#include <functional>
#include <map>
#include <string>
#include <string_view>
@@ -40,24 +41,23 @@
~FakePrefs();
// PrefsInterface methods.
- bool GetString(const std::string& key, std::string* value) const override;
- bool SetString(const std::string& key, std::string_view value) override;
- bool GetInt64(const std::string& key, int64_t* value) const override;
- bool SetInt64(const std::string& key, const int64_t value) override;
- bool GetBoolean(const std::string& key, bool* value) const override;
- bool SetBoolean(const std::string& key, const bool value) override;
+ bool GetString(std::string_view key, std::string* value) const override;
+ bool SetString(std::string_view key, std::string_view value) override;
+ bool GetInt64(std::string_view key, int64_t* value) const override;
+ bool SetInt64(std::string_view key, const int64_t value) override;
+ bool GetBoolean(std::string_view key, bool* value) const override;
+ bool SetBoolean(std::string_view key, const bool value) override;
- bool Exists(const std::string& key) const override;
- bool Delete(const std::string& key) override;
- bool Delete(const std::string& key,
+ bool Exists(std::string_view key) const override;
+ bool Delete(std::string_view key) override;
+ bool Delete(std::string_view key,
const std::vector<std::string>& nss) override;
- bool GetSubKeys(const std::string& ns,
+ bool GetSubKeys(std::string_view ns,
std::vector<std::string>* keys) const override;
- void AddObserver(const std::string& key,
- ObserverInterface* observer) override;
- void RemoveObserver(const std::string& key,
+ void AddObserver(std::string_view key, ObserverInterface* observer) override;
+ void RemoveObserver(std::string_view key,
ObserverInterface* observer) override;
private:
@@ -92,24 +92,25 @@
static std::string GetTypeName(PrefType type);
// Checks that the |key| is either not present or has the given |type|.
- void CheckKeyType(const std::string& key, PrefType type) const;
+ void CheckKeyType(std::string_view key, PrefType type) const;
// Helper function to set a value of the passed |key|. It sets the type based
// on the template parameter T.
template <typename T>
- void SetValue(const std::string& key, T value);
+ void SetValue(std::string_view key, T value);
// Helper function to get a value from the map checking for invalid calls.
// The function fails the test if you attempt to read a value defined as a
// different type. Returns whether the get succeeded.
template <typename T>
- bool GetValue(const std::string& key, T* value) const;
+ bool GetValue(std::string_view key, T* value) const;
// Container for all the key/value pairs.
- std::map<std::string, PrefTypeValue> values_;
+ std::map<std::string, PrefTypeValue, std::less<>> values_;
// The registered observers watching for changes.
- std::map<std::string, std::vector<ObserverInterface*>> observers_;
+ std::map<std::string, std::vector<ObserverInterface*>, std::less<>>
+ observers_;
DISALLOW_COPY_AND_ASSIGN(FakePrefs);
};