武器大师e技能:初学C++关于字符（串）录入的问题。

“让函数只返回接收键盘输入的double数值，对于其他类型数值都不接收，并给出提示。”

我认为最好是写一个接受字符串参数、返回 double、并利用C++ 异常处理报错的函数。
我写的 strToDouble 就是这么一个函数。若参数通过所有检测，strToDouble 便利用 stringstream 类把参数转换成 double。

下面是完整的代码。代码并不复杂，而且有文档。
main( ) 里有调用该函数的例子。

#include<iostream>
#include<string>
#include<sstream>
#include<cctype> // isdigit( )

using namespace std;

/*
* Thrown by strToDouble.
*/
class NumberFormatException {
    // The name of this class is all that is needed.
};

/*
* Returns a double corresponding to the number represented by s if the
* number is in valid double format; else throws NumberFormatException.
*/
double strToDouble( const string& s ) throw( NumberFormatException ) {

    //
    // Validation.
    //
    string copy = s;

    // Erase the optional decimal point.
    size_t dotpos = copy.find( '.' );
    if( dotpos != string::npos )
        copy.erase( dotpos, 1 );

    // Erase the optional sign that is allowed only as the first char.
    if( copy[ 0 ] == '+' || copy[ 0 ] == '-' )
        copy.erase( 0, 1 );

    // At this point, copy should contain only digits.
    for( size_t i = 0; i < copy.size( ); ++i )
        if( ! isdigit( copy[ i ] ) )
            throw NumberFormatException( );

    // At least one digit is required.
    if( ! copy.size( ) ) throw NumberFormatException( );

    //
    // Validation completed. Do conversion.
    //
    stringstream ss( s );
    double d;
    ss >> d;

    return d;
}

int main( ) {

    cout << "Enter a valid number (sign and decimal point is allowed):\n";

    string s;
    double d;

    for(;;) {
        cin >> s;

        try {
            d = strToDouble( s );
            break;
        }
        catch ( NumberFormatException& ) {
            cout << s << " isn't a valid number. Try again.\n";
        }
    }

    cout << d << " is valid. Thank you!\n";

    return 0;
}

你可以这样：
cout<<"......:";
double next;
cin>>next;
bool more=true;
while(more)
{
if(cin.fail())
{cout<<"please input again:";
cin>>next;
}
else
more=false;
}//go on;

cin和cout可以直接处理double类型的数据