迭代器是否具有默认值？

如何解决迭代器是否具有默认值？

C ++标准是否保证以下代码正确？

import plotly.graph_objects as go
import numpy as np

np.random.seed(42)
y0 = np.random.randn(50)
y1 = np.random.randn(50) + 1
y2 = np.random.randn(50) + 2 # shift mean
x0 = 'Sample A'
x1 = 'Sample B'
x2 = 'Sample C'

fig = go.figure()
fig.add_trace(go.Scatter(
    y=[x0]*len(y0),x=y0,mode='markers',name='Sample A',legendgroup='A',showlegend=False
))
fig.add_trace(go.Scatter(
    y=[x1]*len(y1),x=y1,name='Sample B',legendgroup='B',showlegend=False
))
fig.add_trace(go.Scatter(
    y=[x2]*len(y2),x=y2,name='Sample C',legendgroup='C',showlegend=False
))

fig.add_trace(go.Box(
    x=y0,marker_color = 'indianred',Boxpoints=False,yaxis='y2'
))
fig.add_trace(go.Box(
    x=y1,marker_color = 'lightseagreen',yaxis='y2'
))
fig.add_trace(go.Box(
    x=y2,marker_color = 'orange',yaxis='y2'
))

fig.update_layout(
    yaxis2=dict(
        matches='y',layer="above traces",overlaying="y",showticklabels=False
))

fig.show()

（至少它与同时具有#include <iostream> #include <vector> std::vector<int>::iterator i = {}; int main() { if (i == std::vector<int>::iterator{}) { std::cout << "it is empty" << std::endl; } return 0; } 和std::vector<int>的MSVC2017一起使用）