例如，我想声明指向数组的指针并且不想初始化它。然后，我想将它传递给一个函数并在函数中完全初始化它。我如何在 C++ 中做到这一点？
void F(int *B, const int& N) {
B = new int[N];
for (int i = 0; i < N; ++i) B[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(*B, N);
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
它引起/产生错误对应于 B 没有指向点的引用，因此他无法初始化数组。
错误：进程已完成，退出代码为 11
答案 0 :(得分：4)
您传递的指针是原始指针的副本。当您在 data_prep.miRNA.complete.plot <- structure(list(miRNA = c("hsa-let-7a-3p", "hsa-let-7a-3p", "URS0000681820-snRNA",
"URS0000681820-snRNA"), ID = c("86", "175", "9873", "9989"),
value = c(6.11215002618037, 5.03074511800067, 8.5907457800894,
2.25990049836193), Alder = c(75L, 68L, 69L, 55L), Kjonn = c("Mann",
"Mann", "Mann", "Kvinne"), BoneDisease = c("utenBD", "BD",
"utenBD", "BD"), ISS_Stadium_inndeling_Dec2018 = c("3", "1",
"3", "3"), ProgessionDate = c("12/12/2019", "17/06/2014",
"29/12/2017", "14/11/2019"), Progression = c(0L, 1L, 1L,
1L), DeadTimepoints = c("2015-11-24", "2014-10-06", "2018-02-04",
"2020-09-01"), Status = c(1L, 1L, 1L, 1L), TimeDiff = c(71.0416666666667,
601.958333333333, 2796.04166666667, 1903), Relapse = c("2019-12-12",
"2014-06-17", "2017-12-29", "2019-11-14"), TimeDiffRelapse = c(1550.04166666667,
490.958333333333, 2759.04166666667, 1611.04166666667), RNAType = c("MicroRNA",
"MicroRNA", "snRNA", "snRNA")), row.names = c(NA, -4L), class = c("tbl_df",
"tbl", "data.frame"))
types <- c("T", "F")
ui.miRNA <- dashboardPage(
# Application title
dashboardHeader(title=h4(HTML("Plot"))),
dashboardSidebar(
selectInput(
"MicroRNA", "MicroRNA",
choices = data_prep.miRNA.complete.plot %>% filter(RNAType == "MicroRNA") %>% distinct(miRNA) %>% pull(miRNA)
),
selectInput(
"snRNA", "Other sncRNA",
choices = data_prep.miRNA.complete.plot %>% filter(RNAType == "snRNA") %>% distinct(miRNA) %>% pull(miRNA)
),
materialSwitch(inputId = "pval1", label = "MicroRNA P-value"),
materialSwitch(inputId = "pval2", label = "Other sncRNA P-value"),
materialSwitch(inputId = "risk1", label = "MicroRNA Risk table"),
materialSwitch(inputId = "risk2", label = "Other sncRNA Risk table")
),
dashboardBody(
sliderInput("obs1", "Quantiles MicroRNA",
min = 0, max = 1, value = c(0.5, 1)
),
sliderInput("obs2", "Quantiles other sncRNA",
min = 0, max = 1, value = c(0.5, 1)
),
tabsetPanel(
tabPanel("Plot",
plotOutput("myplot1", width = "400px", height = "400px"),
plotOutput("myplot2", width = "400px", height = "400px"))
)
)
)
server <- function(input, output, session) {
output$myplot1 <- renderPlot({
req(input$MicroRNA)
df.t.sub <- data_prep.miRNA.complete.plot %>% filter(RNAType == "MicroRNA" & miRNA %in% input$MicroRNA)
lower_value <- input$obs1[1]
upper_value <- input$obs1[2]
fitSurv <- survfit(Surv(TimeDiff, Status) ~ cut(value, quantile(value, probs = c(0, lower_value, upper_value)), include.lowest=TRUE),data = df.t.sub)
new_env <- environment()
new_env$value <- df.t.sub$value
new_env$TimeDiff <- df.t.sub$TimeDiff
new_env$Status <- df.t.sub$Status
new_env$lower_value <- lower_value
new_env$upper_value <- upper_value
ggsurvplot(fitSurv,
new_env,
title="MicroRNA", xlab="Time (Yrs)",ylab="Survival prbability",
font.main = 8,font.x = 8,font.y = 8,font.tickslab = 8,font.legend=8,pval.size = 3,
font.title = c(16, "bold"),pval.coord = c(1000,1),size=0.4,legend = "right",
censor.size=2,break.time.by = 365,pval =input$pval1,fontsize =2,
palette = c("blue", "red"),ggtheme = theme_bw(),risk.table = input$risk1,xscale=365.25,
xlim=c(0,7*365),legend.title = "Expression",legend.labs = c("Low","High"))
})
output$myplot2 <- renderPlot({
req(input$snRNA)
df.t.sub <- data_prep.miRNA.complete.plot %>% filter(RNAType == "snRNA" & miRNA %in% input$snRNA)
lower_value <- input$obs2[1]
upper_value <- input$obs2[2]
fitSurv <- survfit(Surv(TimeDiff, Status) ~ cut(value, quantile(value, probs = c(0, lower_value, upper_value)), include.lowest=TRUE),data = df.t.sub)
new_env <- environment()
new_env$value <- df.t.sub$value
new_env$TimeDiff <- df.t.sub$TimeDiff
new_env$Status <- df.t.sub$Status
new_env$lower_value <- lower_value
new_env$upper_value <- upper_value
ggsurvplot(fitSurv,
new_env,
title="sRNA", xlab="Time (Yrs)",ylab="Survival prbability",
font.main = 8,font.x = 8,font.y = 8,font.tickslab = 8,font.legend=8,pval.size = 3,
font.title = c(16, "bold"),pval.coord = c(1000,1),size=0.4,legend = "right",
censor.size=2,break.time.by = 365,pval =input$pval2,fontsize =2,
palette = c("blue", "red"),ggtheme = theme_bw(),risk.table = input$risk2,xscale=365.25,
xlim=c(0,7*365),legend.title = "Expression",legend.labs = c("Low","High"))
})
}
shinyApp(ui.miRNA, server)
中修改 B 时，F i B 保持不变。
需要通过引用传递指针或者传递一个指向指针的指针：
main()或
void F(int*& B, const int& N) {
B = new int[N];
for (int i = 0; i < N; ++i) B[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(B, N); //no operator here, just pass the pointer by reference
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
此外，由于您决定使用手动内存管理，您还需要负责释放通过 void F(int** B, const int& N) {
*B = new int[N];
for (int i = 0; i < N; ++i) (*B)[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(&B, N); //need address-of operator
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
获得的内存。如果您使用 new 而不是原始指针（或 std::vector 或其他容器）作为容器，则不需要这样做。
std::unique_ptr答案 1 :(得分：3)
你至少有两个问题。
您使用 *B 的通话中的第一个。 *B 与 B[0] 相同，这意味着您试图将单个 int 元素传递给函数。但由于 B 是空指针，取消引用将导致未定义行为。
正确（适合显示的函数签名）应该是简单的B：
F(B, N);
第二个问题是默认情况下将参数传递给函数是按值完成的。这意味着 B 的值被复制到函数 B 内的局部变量 F 中。
当你在函数中赋值给 B 时，你只会改变本地副本，main 函数中的原始值不会被修改。
要解决这个问题，您需要通过 reference 传递 B：
void F(int*& B, const int N) { ... }