C ++警告：不推荐从字符串常量转换为'char *'[-Wwrite-strings] [英] C++ warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]

问题描述

我使用gnuplot在C ++中绘制图形。图表按预期绘制，但在编译期间有警告。警告是什么意思？

 警告：不推荐从字符串常量转换为char *[-Wwrite-strings] 
  

这是我使用的函数：

  void plotgraph（double xvals []，double yvals []，int NUM_POINTS）
 {
 char * commandsForGnuplot [] = {set title \Probability Graph \\，
plot'data.temp'用行}; 
 FILE * temp = fopen（data.temp，w）; 
 FILE * gnuplotPipe = popen（gnuplot -persistent，w）; 
 int i; 
 for（i = 0; i  {
 fprintf（temp，％lf％lf \\\
，xvals [i]，yvals [i]） ; 
 //将数据写入临时文件
} 
 for（i = 0; i  {
 fprintf（gnuplotPipe， ％s \\\
，commandsForGnuplot [i]）; 
 //逐个向gn uplot发送命令。 
} 
 fflush（gnuplotPipe）; 
} 
  

解决方案

字符串文字是 array of const char ，我们可以从草稿C ++标准部分 2.14.5 看到这一点， > emphasis mine ）：

普通字符串文字和UTF-8字符串文字也被称为窄字符串文字。窄字符串字符串具有类型n const char的数组，其中n是下面定义的字符串的大小，并且具有静态存储持续时间（3.7）。

因此此更改将删除以下警告：

  commandsForGnuplot [] = {set title \Probability Graph \，plot'data.temp'with lines}; 
 ^^^^^ 
  

注意，允许一个< 是 来指向 const //en.wikipedia.org/wiki/Undefined_behavior\">未定义的行为。我们可以通过转到 7.1.6.1 部分cv-qualifiers 部分：

除了任何类成员声明mutable（7.1.1）可以被修改，
任何尝试修改const对象在其生命周期（3.8）结果
in undefined行为。

和 2.14.5 em>它说：

是否所有字符串字面值是不同的（即存储在
非重叠对象） 。
尝试修改字符串文字的效果未定义。

I am using gnuplot to draw a graph in C++. The graph is being plot as expected but there is a warning during compilation. What does the warning mean?

warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]

This is the function I am using:

void plotgraph(double xvals[],double yvals[], int NUM_POINTS)
{
    char * commandsForGnuplot[] = {"set title \"Probability Graph\"", 
        "plot     'data.temp' with lines"};
    FILE * temp = fopen("data.temp", "w");
    FILE * gnuplotPipe = popen ("gnuplot -persistent ", "w");
    int i;
    for (i=0; i < NUM_POINTS; i++)
    {
        fprintf(temp, "%lf %lf \n", xvals[i], yvals[i]); 
        //Write the data to a te  mporary file
    }
    for (i=0; i < NUM_COMMANDS; i++)
    {
        fprintf(gnuplotPipe, "%s \n", commandsForGnuplot[i]); 
        //Send commands to gn  uplot one by one.
    }
    fflush(gnuplotPipe);
}

解决方案

String literals are an array of const char, we can see this from the draft C++ standard section 2.14.5 String literals which says (emphasis mine):

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type "array of n const char", where n is the size of the string as defined below, and has static storage duration (3.7).

so this change will remove the warning:

const char * commandsForGnuplot[] = {"set title \"Probability Graph\"", "plot     'data.temp' with lines"};
^^^^^

Note, allowing a non-const char* to point to const data is a bad idea since modifying a const or a string literal is undefined behavior. We can see this by going to section 7.1.6.1 The cv-qualifiers which says:

Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior.

and section 2.14.5 String literals which says:

Whether all string literals are distinct (that is, are stored in nonoverlapping objects) is implementation defined. The effect of attempting to modify a string literal is undefined.

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