std::initializer_list in C++

0001-01-01 约 328 字预计阅读 1 分钟

Table of Contents

1 Referencing local array
2 The cost of copying elements

本文为摘录(或转载)，侵删，原文为： https://www.cppstories.com/2023/initializer_list_improvements/

1 Referencing local array

std::initializer_list 在使用中会转化成为常量对象的匿名局部数组：

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std::initializer_list<int> wrong() { // for illustration only!
    return { 1, 2, 3, 4};
}
int main() {
    std::initializer_list<int> x = wrong();
}

The above code is equivalent to the following:

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std::initializer_list<int> wrong() {
    const int arr[] { 1, 2, 3, 4}
    return std::initializer_list<int>{arr, arr+4};
}
int main() {
    std::initializer_list<int> x = wrong();
}

如果尝试编译的话，会出错：

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GCC:
warning: returning temporary 'initializer_list' does not extend the lifetime of the underlying array [-Winit-list-lifetime]
    5 |     return { 1, 2, 3, 4};

Or in Clang:

<source>:5:12: warning: returning address of local temporary object [-Wreturn-stack-address]
    return { 1, 2, 3, 4};

结论：

std::initializer_list 是一种 “view” type
他引用一个局部的常量数组 it references some implementation-dependent and a local array of const values.
Use it mainly for passing into functions when you need a variable number of arguments of the same type.
If you try to return such lists and pass them around, then you risk lifetime issues

2 The cost of copying elements

通过初始化列表来传递参数很方便，但最好知道，当向 vector 中传递对象时候，vector 中的每个元素都会进行一次拷贝构造，应该了解其代价。

目录

std::initializer_list in C++

1 Referencing local array

2 The cost of copying elements