C++常用类型转换

1.QString转string

QString  qstr = "test";
string s = qstr.toStdString();

2.string转QString

string s = "test";
QString qstr = QString::fromStdString(s);

3.QString转char *

方法1（推荐）：

QString  qstr = "test"; 
char * filename = qstr.toUtf8().data();

方法2：

QString  qstr = "test"; 
char * filename = qst.toLocal8Bit().data();

4.char *转QString

char *ch="test";
QString str(ch);   // Qt5     
QString str = QString::fromUtf8(ch));    //  Qt4

5.QString转const char *

方法1：

QString  qstr = "test"; 
char* filename = qstr .toStdString().c_str();

6.float转string

stringstream ss;
float b = 10.1;
ss << b;
string p;
ss >> p;
cout << p;

注意：如果需要多个float转成string，stringstream的变量ss貌似无法共用，例如下面的例子中只有thresh1能成功输出。thresh2和thresh3是空的。

stringstream ss;
float thresh1 = 10.1, thresh2 = 20.1, thresh3 = 30.1;
string p1, p2, p3;
ss << thresh1;
ss >> p1; cout << p1;
ss << thresh21;
ss >> p2; cout << p2;
ss << thresh3;
ss >> p3; cout << p3;

如果需要多个float转成string，可以用多个stringstream变量，如下：

stringstream ss1, ss2, ss3;
float thresh1 = 10.1, thresh2 = 20.1, thresh3 = 30.1;
string p1, p2, p3;
ss1 << thresh1;
ss1 >> p1; cout << p1;
ss2 << thresh21;
ss2 >> p2; cout << p2;
ss3 << thresh3;
ss3 >> p3; cout << p3;

方法2：借助float转QString，再QString转string

7.float转QString

float thresh1 = 10.1, thresh2 = 20.1, thresh3 = 30.1;
QString  qstr = QString("%1+%2 %3").arg(thresh1).arg(thresh2).arg(thresh3);