C++ 警告：不推荐将字符串常量转换为 'char*' [-Wwrite-strings]
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C++ warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
提问by Sagar
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?
我正在使用 gnuplot 在 C++ 中绘制图形。该图正在按预期绘制，但在编译期间出现警告。警告是什么意思？
warning: deprecated conversion from string constant to ‘char*' [-Wwrite-strings]
This is the function I am using:
这是我正在使用的功能：
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);
}
回答by Shafik Yaghmour
String literals are an array of const char, we can see this from the draft C++ standard section 2.14.5String literalswhich says (emphasis mine):
字符串文字是一个const char 数组，我们可以从 C++ 标准草案部分2.14.5字符串文字中看到这一点，它说（强调我的）：
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).
普通字符串文字和 UTF-8 字符串文字也称为窄字符串文字。窄字符串字面量的类型为“array of n const char”，其中 n 是如下定义的字符串大小，并且具有静态存储持续时间 (3.7)。
so this change will remove the warning:
因此此更改将删除警告：
const char * commandsForGnuplot[] = {"set title \"Probability Graph\"", "plot 'data.temp' with lines"};
^^^^^
Note, allowing a *non-const char** to point to constdata is a bad idea since modifying a constor a string literalis undefined behavior. We can see this by going to section 7.1.6.1The cv-qualifierswhich says:
请注意，允许 *non-const char** 指向const数据是一个坏主意，因为修改const或字符串文字是未定义的行为。我们可以通过转到7.1.6.1cv-qualifiers部分来看到这一点，其中说：
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.
除了任何声明为 mutable (7.1.1) 的类成员都可以修改之外，任何在其生命周期 (3.8) 期间修改 const 对象的尝试都会导致未定义的行为。
and section 2.14.5String literalswhich says:
和部分2.14.5字符串文字它说：
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.
是否所有字符串文字都是不同的（即存储在非重叠对象中）是实现定义的。尝试修改字符串文字的效果是未定义的。
