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c++ - c++11 中首选的初始化方式

转载 作者：塔克拉玛干 更新时间：2023-11-02 23:14:09 25 4
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int i = 0; // (a) Old C style should I use it?
int i{0}; // (b) Brace direct init
int i{}; // (c) Same as (b)
int i = {0}; // (d) as (b)
int i = {}; // (e) as (c)
auto i = 0; // (f) auto = int in this case.
auto i = int{0}; // (g) auto = more specific.
auto i = int{}; // (h) same as above (g)

使用哪一个？萨特说使用:

int i = 0;
auto i = 0;

为什么不呢:

int i = {0};
auto i = int{0};

在某些情况下我应该去掉“=”吗:

int i{0};
auto i{0}; // i is not what some might expect in this case. So I would prefer using "=" everywhere possible like int i = {0}; ...

编辑:这就是我的目标，在我看来这是最一致的:

rectangle       w   = { origin(), extents() }; 
complex<double> c   = { 2.71828, 3.14159 }; 
mystruct        m   = { 1, 2 }; 
int             a[] = { 1, 2, 3, 4 };
vector<int>     v   = { 1, 2, 3, 4 };
point           p   = {}; // Default initializes members
int             i   = {0}; // Checked assembly for this and it's binary the same as int i{0}; could be written also as int i = {};
string          s   = {""}; // Same as string s = {}; (OR) string s;

现实生活中的例子:

std::string       title              = { pt.get<std::string>("document.window.title") };
const std::string file               = { R"(CoreSettings.xml)" };
int_least64_t     currentTick        = { 0 }; // (OR) int_least64_t currentTick = {};
bool              isRunning          = { false }; // (OR) bool isRunning = {};
App*              app                = { nullptr }; // (OR) App* app = {};
Event             event              = {};
double            detectedFrameRate  = { 1000000000.0 / (swapIntervalDeltaCumulative / 20.0) };
double            precision          = { static_cast<double>(boost::chrono::high_resolution_clock::period::num)
                                           / boost::chrono::high_resolution_clock::period::den };
auto              timeSpan           = boost::chrono::duration_cast<boost::chrono::nanoseconds>(nowTime - startTime);

替代方案是:

std::string       title             { pt.get<std::string>("document.window.title") };
const std::string file              { R"(CoreSettings.xml)" };
int_least64_t     currentTick       { 0 }; // (OR) int_least64_t currentTick{};
bool              isRunning         { false }; // (OR) bool isRunning{};
App*              app               { nullptr }; // (OR) App* app{};
Event             event             {};
double            detectedFrameRate { 1000000000.0 / (swapIntervalDeltaCumulative / 20.0) };
double            precision         { static_cast<double>(boost::chrono::high_resolution_clock::period::num)
                                        / boost::chrono::high_resolution_clock::period::den };
auto              timeSpan          = boost::chrono::duration_cast<boost::chrono::nanoseconds>(nowTime - startTime);

如果不使用大括号，它会很难看或容易出错:

int_least64_t     currentTick        = 0; // C style - changed this from double to int recently and compiler did not complain so I had something like int_least64_t currentTick = 0.0; ugly!
bool              isRunning          = false; // C style
App*              app                = nullptr; // C mixed with C++11 style;
Event             event; // might not be initialized by all compilers
int               someInt            = func(); // func() returns double no error but narrowing.

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